I almost always hear this while still parked at the gate, I always assumed it was because the number of boarded pax and the manifest don't match and they are trying to find someone. For example the GA came on my YOW to EWR last week and they made this announcement "if you are on board please identify yourself using the flight attendant call button" and I'm sitting in row 1 thinking ... this CRJ-100 is the size of my living room - I'm pretty sure they could just stand up or raise their hand

I knew it!