Another example to support your view; Lower and Middle Staple Tors. Both Hybrid boxes. GCT4CE
Both had geocaches when I walked around there a couple of months ago. I tried both, one was obviously missing (clear clue), one I couldn’t find but could have been there. The owner admitted they weren’t able to do maint so archived the one that was definitely missing. End of a 6 year old cache 🙁 (They may be persuaded to unarchive if anyone wants to adopt it, sadly it’s the wrong side of the moor for me)
In both cases, there have been a lot of DNF *AND* Found logs. The latter, almost certainly from people finding nearby letterboxes and not realising. Given how many rock solid finds there were earlier, I suspect it has gone but without the CO available to verify what is and what isn’t their cache – the only way to distinguish a Hybrid Geocache from a bog standard letterbox is some form of identification; such as the stash card you suggested.
That said, I put out the Hennock series without them. I did mean to, and actually designed a special one, but I had a free day to lay it before I went back to work where the laminator was and put them out without. I’ll include them when I do a maint-circuit though. Most have “Geocache – Harmless” tz-labelled on the outside as per the guidelines though.