I would simply remind her that your room is off-limits, and that you expect her to respect your privacy.
Then, when she's not home, YOU go thru your office (and your car would be wise as well), and -- much as you would "child-proof" a home when a toddler arrives -- "WW-proof" it. Make sure there's nothing you wouldn't want her to find.
Finally, if you want to have some fun, two ideas:
1. take your own business card, if you have one, and write on the back of it "Kim 5pm Fri." and nothing else. Leave it where you might normally empty your pockets at the end of the day, but don't be TOO obvious about it.
(alt. version: add the name of a LOCATION to the note -- like a pub -- and show up 30 minutes before the appointed time, park your car and wait for your wife to show up, LOL. Do NOT answer your cellphone when she calls you).
2. Put a piece of paper in your desk, folded in half, that says "(WIFE) -- I asked you not to look thru my things. Really, it is SOOOOO petty and unattractive! :o)" -- with the smiley face.
She's fishing. Be sure you have no bait she can find.
