He has a kind of satisfying expression like he just grope Sango's or a beutiful woman's ass and didn't get slapped for it (heh like THAT will ever happen ) !!! Or he accomplished killing a demon!!! Or something like that!!!

It DID happen, he was "helping" a woman who had a cramp in her back or soemthing and rubbed her butt several time.

Wait! Now I remember this episode <--- episode 118. It's the episode where Sango and Miroku were in Mt.Hakurei and Miroku and Sango got separted becuase of Kagura and Kagura also reversed Sango's Hiraikotsu attack and she fainted. Then Miroku had to save her by using Kazaana to suck up the youkai and saimyosho unfortunately. With Sango over his shoulder, he fell to the ground and when Sango woke up, she saw Miroku on her. Miroku wakes up told her to go on ahead, but said she wanted to die with him. Aw...