A mystery intruder has been plundering cereal and sleeping on the couches for weeks at one of Sydney's marketing agencies.

Staff at the The Marketing Store were perplexed and slightly amused when, almost daily, they would arrive at their Ultimo office to find a trail left by their unwanted nocturnal visitor.

Caught on cerealcam ... the brekkie bandit.

"My office was the starting point for it,'' said the agency's Asia Pacific president Doug Chapman.

"I'd come in and I'd notice somebody had been sleeping on the lounge or doing something in my office.