Are you afraid of her finding more?
If that's what's behind your question, then I second SlowUptake's suggestion to be open about this with your wife. "BW, I thought I deleted all photos/emails of AP off the computer. Since you found [the email & photo in question], apparently they are still accessible. What would you like me to do about this?" If she wants to use her method to look for more, allow her to do so and then allow her to delete them if she wishes, etc. Or if she prefers you to reformat the computer, do that. Me: XWS, 30s, 5-month EA/PA in '09-'10
Husband: XBS, 40s
Married 2.5 years