From "Mr. Monk goes on the Run" when asked what he exchanged with a homeless man in order to obtain a coat.
Adrian Monk: I can't tell you, Natalie. Don't make me tell you.
I always thought it was probably something that caused him to get dirt on himself.
In "Mr. Monk's Favorite Show", Monk volunteers to bodyguard his childhood idol Christine Rapp, whose tell-all autobiography got her multiple death threats or something like that. Natalie reads it, and it's so bad that she has to tear pages out and still won't let Monk read it, the implication being that either described or outright illustrated are some very very horrible sex acts. Later Monk is helping her secure her bedroom, and sees a mirror on her ceiling. He comments that it makes a great security feature, to which she replies something like "That's not what it's for." It's actually for rich people to admire themselves mid-coitus.
Not the first time a "mirror on the ceiling" reference was made. It happened in "Mr. Monk Meets the Playboy," when Monk sees Dexter Larsen's bedroom:
Adrian Monk: Look at that. Thereís a mirror above the bed. Why would he need a mirror on the ceiling?
Sharona Fleming: Donít think about that now. Not now.