"I was cleaning my underpants!"
A stunned security guard stumbled onto the man in the middle of a compromising act with the cleaner, which has a large smiley face painted on its front and a hose protruding from its "nose".
According to the Sun, the contractor was supposed to be locking up the building site near the Great Ormond Street Children's Hospital where his firm is refurbishing administration offices.
The security guard, suitably horrified, told the man to "clean himself and the hoover" before asking him to leave and informing his bosses.
When later questioned by his employers, the man said he was vacuuming his underpants, which was "a common practice in Poland". He has since been fired.
The man's employers, HG Construction, told The Sun: "That behaviour is not acceptable, though it gave a few people a laugh".