News that artefacts were stolen from the British Museum has led to interest in what barriers exist to stop people from taking some of the world’s greatest treasures.

Earlier this week, the museum sacked a member of staff after treasures were reported missing, and the Metropolitan Police opened an investigation.

The items that were stolen had been used for research and had not recently been on display.

An art recovery expert tells the BBC that, despite security, museum thefts are common, and the onward journey of these stolen items can range from selling them in the black market to melting antiquities down. Sometimes, they are sold for far less than they were originally worth.

Artefact thefts happen “every single day around the world,” according to Christopher Marinello, lawyer and founder of Art Recovery International. His organisation specialises in locating and recovering stolen works of art worldwide.

But Mr Marinello says it is “shocking” when large institutions, such as the British Museum, are caught off-guard.

The British Museum has experienced thefts before. In 2002, it launched a security review after a 2,500-year-old Greek statue was stolen by a member of the public.

The thief took the 12cm-high (4.7in) marble head and left with it undetected.

At the time, the museum said no permanent guard had been on duty in the Greek Archaic Gallery – the part of the museum from which the statue was stolen – despite it being open to the public.

The museum also said it had reviewed its security for its collections, and invested significantly, after a Cartier ring worth £750,000 went missing from its heritage asset collection.

It had not been on public display. The ring was reported missing to police in 2011, but details did not emerge publicly until 2017.

While thefts from museums might conjure images of items pinched from glass cases, a large proportion of museum treasures are actually stored behind closed doors.

The Natural History Museum, for example, has 80 million objects in its collection – and scientists use them to answer key questions about the past, present and future. Only a “tiny fraction” of them are on public display, the museum says on its website.

The British Museum’s collection consists of at least eight million objects, of which roughly 80,000 are on public display, according to a fact sheet published by the museum in 2019.

“A lot of the focus goes onto security measures for public-facing displays” rather than stored collections, criminology professor Emmeline Taylor tells BBC News.

Museums and galleries in the UK need to abide by certain security requirements to be accredited by the Arts Council, the national development agency for creativity and culture.

The council says institutions should get security advice for all the buildings and sites that the museum occupies – particularly arrangements for staff, volunteers and visitors, stored and displayed collections, buildings, and sites.

Alice Farren-Bradley manages the global Museum Security Network, which shares information on security, common threats and risks with its roughly 1,500 members.

She says artefacts that are the most fragile and unlikely to go on display or on loan are kept in “deep storage”. Some are in general storage and others are in study collections, accessible upon request by academics.

Ideally, every item should be inventoried with a detailed description, number and photographs from multiple angles, she says. But due to their age and size, most collections do not have 100% of items catalogued.

Other protections include items being kept in locked storage rooms, with electronic sensors on entrances to such areas, CCTV, and systems that record staff movements via their swipe cards, which grant them degrees of access.

But these systems only work if there are security teams monitoring them and picking up on discrepancies, and if the data needed is still accessible, she says.

And, she points out: “If you don’t have a complete inventory, everything else – all the other security measures – they are only going to get you so far in terms of protecting the collection.”

Ms Farren-Bradley warns there could be instances of insider theft that museums are not even aware of, as it can be months or years before they realise items are missing.

She has seen situations where an insider is not only removing artefacts, but amending records so that if someone checks the system it says the item is on loan, or was never part of the collection.

And she is aware of one case where someone was taking items out of cases, and rearranging the display so it was not immediately obvious something was missing.

Asked about other security measures museums rely on, she says some have bag checks, metal detecting equipment and X-ray machines for staff and visitors.

Others have a whistleblowing system where personnel at any level can flag a concern.

“Museums need a policy or a system in place that can listen to people’s concerns, because most of the time human behaviour is one of the best detectors of some sort of problem,” she says.

But this is only part of the picture – she says museums also need the resources to investigate such reports, with security “so closely” tied to funding.

The Art Loss Register, which describes itself as the world’s largest private database of lost, stolen and looted art, antiques and collectibles, currently lists around 700,000 items.

The London-based organisation helps people check the provenance of items before buying or handling them. It also provides free assistance to law enforcement agencies and nation states.

Some 450,000 searches are carried out each year on items up for sale, on behalf of governments and law enforcement bodies, auction houses, art fairs, dealers, banks and lenders, museums and galleries, pawnbrokers and private individuals.

Mr Marinello, from Art Recovery International, says his organisation is notified of thefts by galleries and museums once an inventory has been established – which lists missing objects.

Prof Taylor says once rare items are stolen, it becomes “a race against time” for those trying to sell it on.

Sometimes, items worth huge sums end up being sold on for relatively little. In 2005, for example, a Henry Moore sculpture worth £3m was stolen and police believe it was melted down and sold for less than £1,500.

Reputable auction houses will not sell items that they know are stolen – they will hold them, Mr Marinello explains.

But some auction houses, he says, are interested in earning commissions and “get frustrated when someone reports something as stolen in their sale room”.

And then there is the black market. Those trading there will sell items “at a severe discount to collectors and dealers”, Mr Marinello says, “who do no due diligence and really don’t care”.

Prof Taylor, a City University London academic specialising in staff dishonesty, says once thieves obtain an item they will “need to get them [stolen items] out of the country as quickly as possible”.

They will also “need to have a buyer lined up”, she added, as authorities worldwide will have their eyes peeled on the movements of these “hot” items.

Prof Taylor tells the BBC there are three likely motivations behind in-house thefts:

She adds: “Typically with staff dishonesty it starts very small and escalates over time… each time getting a little bolder, taking a little bit more.”

But Ms Farren-Bradley says thefts of artefacts are not always motivated by financial gain.

Some insiders will take items “with no intention of ever selling them, but because they have become attached to the collections and they feel that they appreciate these items more than the institution does”.

Such items will become keepsakes in their homes – with the institution tasked with their protection potentially unaware for years that they are missing.