He was just looking for his t-shirt and he accidentally walked in on you with no pants but only a shirt on

"Hey have you seen my...shirt...?" His eyes trailed you up and down

"That's my shirt. And why are your hands tucked in between your thighs?" You tried to make an excuse but he caught on. "I'm not a fool, dear..."

He walked towards you while unbuckling his belt

(You can easily change how the story goes but either way, have fun.)