I was hypo a few weeks ago while in a PHP. They wanted to put me in inpatient because I have a history of flying way up to mania so they wanted me to talk to the doctor. I had to hide it or I was going to be locked up over Thanksgiving. Lots of deep breaths and reminders, "slow down, escape, you need doc to follow you."
So if I'm hypo, yes I can hide it. If I'm full blown manic, no.