Banksy's spray painting of a Tube carriage showed London Underground was "not safe", the RMT union has said.
A video posted on the artist's Instagram in July showed a man, thought to be Banksy, disguised as a cleaner.
He was wearing personal protective equipment (PPE) and used equipment Tube staff would use to disinfect trains.
Transport for London said the Tube was safe but the RMT, which many Tube staff belong to, said it was not a lapse in security but showed "there wasn't any".
The work by Banksy, called If You Don't Mask, You Don't Get, featured a number of rats in pandemic-inspired poses and wearing face masks.
It was removed by TfL cleaning crews who were said to be unaware it was a Banksy artwork and treated it "like any other graffiti on the network".
'Anything could happen'
The transport authority has now confirmed to the BBC, in response a Freedom of Information Act request, that it had completed an investigation into the incident but would not release its findings because of security concerns and not wanting to encourage copycats.
TfL said the artist carried out the graffiti between Barbican and Paddington on the Hammersmith and City Line between 05:24 and 06:31 on 10 July.
It also confirmed it had no involvement with either the artist, his staff or lawyers.
The footage also showed the artist painted the words "I get lockdown" on a wall right at the end of a station platform. TfL would not say which station it was but confirmed the artist did not go on the tracks.
A spokesperson for the RMT said some stations had no gate-line staff at certain times of the day so people could "literally walk on there with anything they like".
"You can have a coffin under your arm let alone a canister full of paint," he said.
He said the union had been campaigning over staffing levels because the stations were unstaffed for "long periods of time and the end result is anything could happen".
"What happened with a canister full of paint is actually very minor, considering what could have happened."
Safety has been a point of contention between the unions and TfL over the past few years, when the service underwent job cuts as ticket offices were closed.
On its website, TfL says it funds 2,500 police officers to work across London's transport network to keep people safe.
The BBC has approached TfL for a statement.
Security consultant Will Geddes said it was difficult for Tube staff to check everyone to make sure they were who they appeared to be - for example, a cleaner. He said a balance had to be struck as TfL did not want to hold people up by having airport-style security.
"It's a bit of a human rights issue," he said. "If you were to go to the Tube station, you would be very affronted if someone was asking you questions about why you were wearing that particular jacket today, or that hat or whatever it might be."
Mr Geddes said the wearing of face masks on the Tube was a security risk as it meant criminals were harder to spot.
Brian Woodhead, Director of Customer Service for London Underground, said: "The safety of our customers and staff is always our primary concern, and ensuring the security of our network is a key part of that.
"We work closely with our station staff, our trade unions and with partners including the British Transport Police to ensure that the network is as secure as possible."
A Banksy spokesperson said they had no comment to add.