The incident comes a day after Banksy painted swimming piranhas on to a City of London Police sentry box, which was first spotted on Sunday morning.
The translucent fish formed the seventh piece in an animal-themed art series across the capital.
On Monday, the City of London Corporation said the artwork had been removed to a “safe location”.
A spokesperson said: “We have moved the artwork to Guildhall Yard to ensure it is properly protected and open for the public to view safely.
“A permanent home for the piece will be decided in due course.”
On 5 August, a goat appeared on the side of a building near Kew Bridge, followed by an image of two elephants touching trunks on the side of a house in Chelsea on Tuesday.
Three monkeys hanging from a bridge in Brick Lane then drew crowds on Wednesday.
