Eavesdropped, never. ("I ain't been dropping no eaves sir, honest!") Overheard, yes. And it can be so erotic that it drives me crazy. I know he's in there thinking about me and all the things we've done.
I don't think I'd ever listen and/or watch and not tell him. We do it in front of each other almost daily.
But if you're going to watch his personal moments discretely, sassy, I hope that he won't be upset if he finds out. Would it be worth encouraging him to share with you so that you wouldn't need to risk losing his trust?
And for that matter, who am I to try and suggest things to others...I'm a newbie, after all.