The actor recently sat down with Entertainment Weekly for a panel discussion and played Two Truths and a Lie -- a game where a person says two true statements and one lie and others guess which is which -- and he said a woman once left him a brown reminder of their time together.

The way Theo tells the story ... it was back during his university days, and he and this girl hit the town before heading back to his place where they had some "fun" -- you can probably read between the lines on your own.

Theo says his lady friend left in the morning ... but, when he went to his bathroom, he found a gift waiting for him in his bathtub -- a "small turd" as he puts it. Theo says he thought the date went well, so he was a bit miffed to find what she left behind.

No word on how he fished the Hershey's Kiss out of the bath ... but, sounds like he's laughing about it all these years later.

BTW ... not the only bodily functions story Theo's recently told -- revealing earlier this week he actually had a bottle of urine thrown at him while onstage playing with his band. So, the guy's dealt with more than his fair share of excretions.