"I can't say for sure, but he was certainly involved."

The doorman's eyes go wide. "You mean - I - I let that guy in!" He looks distressed. Maybe it was a little tactless to say that outright. You write your phone number on a scrap of paper and hand it to him, doing your best to calm him down, letting him know to call you if he seems HB again. You'll have to run to your office at some point to pick up your mobile.