I don't think they can notice. When I've guested people in the past there was no question about showing a boarding pass. They could've been flying QF or they could've been someone passing by.
What they do know (potentially) is how many guests are let in which could contribute to the overcrowding issue in the lounge.
But again it begs the question would you go through security just to use a lounge and eat some stale bagels at the lounge?
