I understood your question - and your concern. I had hoped the fact that the security feature is a tough nut to crack might be of some consolation. Passcodes are almost impossible to guess, unless the guessER has special knowledge of you. The PIN code has been industry standard in the financial world for decades - but alas, a lot of folks use their birthdate or the digits of their street address.
I searched (which I am pretty good at) and found nothing here in the World of Apple that indicates that there is a log file (which I agree that there MAY be) that is user accessible using the iOS features or any other method. Google was not forthcoming either in the time I devoted there.
It is possible that IF this file exists it may not be permanent so that is a running total of all attempts since one first turned on the phone. It could be like a cookie in a browser or somesuch that resets when one gets by the temporary lock out by correctly entering the passcode. Although robust, iOS is not OSX.
If I had an acquaintance that had possessed my phone for a period of time, I would probably rely on subtle interrogation methods. Most times people are terrible liars. Additionally, I would take actions AS IF the damage had occurred. Better safe than sorry.
best luck
ÇÇÇ