Well yeah, but that would be a complete lie. Not sure you want to go down the road of impersonating a flight crew (in this case off duty)...doesn't sound like a good road, fake phone call or not. Up until that point you could say 'the bag was given to me as a gift, sorry" but once you tell them you are based somewhere, which would be, at the point, more than just implying you work for United, you could be in a pickle. Sounds like super stupid idea.