Depends why in his wallet. If getting something or he asked you to get something, then confront him, it's probly something innocent or he wouldn't be letting ou go in it. If you were snooping, bringing it up may cause even more trust issues because the snooping is breaking trust in itself, and may become an even bigger issue between you than the card you found.
Well, believe in strippers cuz they are real. And just ask him to explain it before accusing him of anything. Because if putting the card in his wallet was innocent, you being accusatory could be received in a bad way.