You need to start being mysterious yourself. Go out with your girlfriends. Go have fun. Don't tell him what you're doing - and if he insists you tell him, just laugh and say "my, aren't we nosy?".
GAL. Get out there. Don't tell him where you're going. Next time he leaves that note on when the dog was walked, call him and tell him that he needs to come home to take care of the dog because you're going out. He'll ask where. Just tell him, "I'm going out."
He'll be back sooner than later. Don't play games. Don't lie to him. Just don't tell him anything. He's doing the same to you, but he is probably lying about where he is staying. From this situation is sounds like a physical affair is a 90% probability. Prepare yourself for that if you haven't already.
Look - he spends 21/30 nights away from home a month? And he says he's staying with friends? And you believe that? Really?
Do you have a car? Can you just drive by his friends house where he is "staying" late one night you know he's not coming home? If not, can you get a friend to take you?
Honey, he's having an affair.
----"Et tu, Brute?"---- me:28 W:24 S4 T:6 M:4 EA Exposed: 5/21/10 Bomb: 6/20/10 (Father's Day) NC w/ OM: 7/10/10 W moved out 8/21/10 http://bit.ly/aOrZne - My sitch