Simultaneous to exposing to OM's wife, if you decide to do that (and whether or NOT you decide to do that), you need to take the note, complete with the date stamp on the envelope. Not a copy, but the ORIGINAL.
Simply take it.
Your wife will be in no position to ask you if you have it, without revealing her little secret. Let her panic when she suddenly can't find it.
During my sitch with my wife last summer, at one point I found that she had photocopied my prescriptions -- the anti-depressants. So I took them, from her purse. She of course couldn't say anything.
A week or so later, I snooped her underwear drawer and found that this time, she had HANDWRITTEN -- in her own handwriting -- what my prescriptions are.
Pretty damning evidence, especially when it's in your own handwriting and has your fingerprints on it.
I took that too.
She couldn't (and didn't) say anything.
A day or so later, I printed out some stuff from the internet about how taking someone's personal medical information without their consent is a federal felony, under the HPAA code. I left that out on the kitchen counter, for her to find, which she did.
Again, she never said anything. But boy, was she ever PANICKED!!!