My protocols on this are;
If I don’t get to carry out a complete search in for whatever reason, I write a note.
If I do not find the cache I write a DNF note unless there is good reason. There is a cache I would dearly love to find, I’ve visited 4 times, dnf’d four times, posted three dnf logs, been polite enough to not ask for assistance from the c/o and writing a fourth dnf log seemed rather pointless as others have found the cache in the meantime. And the c/o is a hard man!!
I do think that cache owners ought to consider granting finds more regularly to people who post dnfs on caches which subsequently turn out to have disappeared. Under no circumstances should a cache owner grant a find to anyone who asks for it but a poster of a dnf log who provides helpful, clear assistance to the c/o ought to be rewarded for it.
The worst case that I have come across of a claimed find that wasn’t, was this: Two years ago, a day before a big wedding in London (so you can find it in my caching record!!), I went caching with my two kids in Snowdonia. We did a series that had been published a couple of months previously and as it was in thoroughly awkward countryside no one had done the series as one walk. One cache we came to had one online log but a blank logbook. The online log had lots of photos but not of this cache. So the cacher had claimed a FTF when he hadn’t found the cache. I claimed the FTF in my log and the local FTF hound did the cache the next day after reading my log. He was not amused! In this case, he was cheated out of a FTF but I agree with the comment that cheaters are cheating only themselves.
I recommend the caches in that area and all caches by that particular owner- all great.