The 32-year-old actor is visiting friends in the English capital but student Marley Spindler, who bumped into the former `Home Alone` star at 2.30am on the streets of the city, first thought the star was a vagrant.
Marley told RadarOnline: "We chatted for about five minutes and he said he was in London seeing friends though he was vacant, speaking very quietly and mellow.
"I didn`t know who he was at first - I thought it was some homeless guy who was scrounging a smoke. "I`m no expert, but he didn`t seem all there."
Macaulay has so far managed to avoid an awkward meeting with his former girlfriend Mila Kunis and her new boyfriend Ashton Kutcher, who are also in London.
Last week Ashton was spotted departing a hotel in Covent Garden just a few streets away from where Macaulay was walking.
The couple are staying in a luxury home in Hampstead, North London while the brunette beauty shoots her latest movie, `Jupiter Ascending` but took some time out for romance last week to attend an opera at the Barbican before having dinner together in Mayfair.