Maybe I am opening myself up for ridicule here, but I keep coded notes on mine and dh's sex life. Not as detailed as I once did, but far more than "normal" people do, I am sure. In my day planner. Which goes EVERYWHERE with me. When I kept a palm, I also kept them there (which means in two separate places, on my person nearly all the time). And that's not including the files on FetilityFriend and my paper versions of my fertility charts (which I also keep), in addition to the "take along" version of my chart and the actual "diary" diary.
As for the note-taking itself, I personally don't think that's weird. It's "his" sex life, and he has the right to record whatever he likes about it on his computer, on his calendar, on a flash drive - whatever. To me, "code" implies that the average person wouldn't be able to understand it (all? some? most?), which to me says that it's for his personal use/reference/to satisfy some internal drive to record information - LOTS of information, about LOTS of things, most likely. Just my perspective here, but sex is private, so I can see the value in keeping it on a flash drive (and thus, 99% of the time "on me") rather than on a computer where someone might stumble across it. If it was in his pants, you can bet that he's LOOKING for it. He may very well be waiting for you to say something about it.
That's my issue, BTW, OCD tendencies that manifest in data hoarding, among other things. It takes a lot of dedication to record every encounter, every position, how long, every location... which also suggests to me either an extremely analytical mind or a compulsion of some sort. Neither are necessarily "wrong" or "bad" - they just ARE.
As for the recording, if I read correctly, then that was some time ago. If this was me, and that issue has been resolved, then I wouldn't like to have assumptions made about the fact that I keep meticulous records of sexual activity turn into suspicions of violations of privacy right off the bat. If you made it known that you don't agree to that type of thing and he hasn't given you any indication that he's done it again, jumping right to that suspicion, while not unreasonable on your part, is not fair, IMO.
What's wrong with just being honest with him? "Hey babe - I found this and plugged it in to make sure it wasn't jacked up and found your lists... Can we talk about it?" Reiterate the previously laid guidelines about recording/video or whatever in the course of the conversation and chat about the things about this that bother you.
If he takes notes for a living, he may be keeping those notes for "fun" - if you're not "one of those people" then that may sound odd, but trust me, note-taking can be fun. Satisfying. Very satisfying... to check boxes or record one more day of info... It may be a control thing for him (projecting here... it is for me) or he may get totally hot by scanning past encounters with you (not projecting here, lol) - and that's NOT WRONG. I have records stretching back nearly 15 years with my dh. Sex with my dh makes me happy. Seeing that much "happiness" in our relationship makes me happy. Not deviantly so, but... secure-happy.
I agree with the pp who suggested that your embarrassment if it was found and decoded be a motivation for being more careful. He may be very upset with himself for "forgetting" to take the flash drive out of his pants before they went through the washer. He may not be interested in talking about it unless you bring it up because of the reaction he's afraid you might have (which a LOT of people share, as evidenced by the "OMG that's totally creepy" sentiments expressed by the majority of posters in this thread). It's embarrassing to have your "private" idiosyncrasies trotted out to be investigated, even by your spouse, especially when it's something that you know is not "normal".
My suggestion is to be honest with him about your feelings. You're entitled to feel how you feel, but recognize that he may be embarrassed (which may manifest as anger) and be respectful of his feelings, too. In any case, I feel that communication is MUCH better than suspicion.
Best of luck.