I’m no prude, I swear, but I just can’t envision any circumstance where I’d say “masturbate” out loud. To anyone. I’ve discussed the subject of course, but the word itself is creepy to me for some reason. Overly clinical. <shrug>
You don’t forget before you masturbate though. If you do, you have no business masturbating in the first place.
The master has spoken. 
Not only should you acknowledge walking in on someone masturbating, you should take a seat and get a good look at his technique to critique or use it to improve your own.
Why stop there? Why not offer to lend a hand? It’s the polite thing to do, anyway!
Whats this, To Catch a Masturbator?
Why dont you take a seat over here…