Log:Didn't find it
Also known as "DNF".
A geocacher that searched for a cache has failed to find it.
The geocacher has searched thoroughly and believes that the cache might be missing or that other circumstances may be involved.
The cache may be indeed missing, there may be changes in the field that prevent access to the cache itself (temporarily), or the cache is fine but the geocacher failed to find it.
Why a geocacher would/should log (or not) a DNF?
It is true that not all searches may be successful. A DNF may be considered a personal failure by some and won't log. Some may exagerate with DNF logs for reasons like "too many muggles, could not search". Some may refrain from logging a DNF because they believe that although the search was thorough, they might have missed something.
A DNF should be logged when the geocacher truly believes that, although he/she failed to find the cache, he/she may have important information to convey to the cache owner this way.
An alternative would be to write a note.
A "Needs maintenance" log is more specific and applies to both found and not found caches.