DNF -the dreaded DNF or Did Not Find. The goal is to go caching and find what you are looking for. Sometimes the cache is missing and sometimes it is there and you just didn't find it. A cache can be missing for various reasons. It is "muggled" or taken by a person that is outside of the geocaching community. A animail could carry it off, a bridge hiding a keybox may be replaced, it falls out of the intended hiding spot and gets put back in the wrong place, it gets washed away, burned up, blown away or picked up and thrown away because it may look like trash. Sometimes it is an evil hide and you may need to read logs, return to the location several times or just get the right angle on it but until the container is in hand and your name on the log then it is a DNF.
No one likes to have a DNF and it is a personal choice wheather you post a DNF log or not. If it is a 1.5 difficulty and a 1.5 terrain with little option for camo or cover then the cache is likely missing. If the cache is a difficutly of 4.5 then it might be there and you just couldn't find it. By posting the DNF log it lets the COs or cache owners know there might be an issue so they can check to see if it is indeed missing and either replace it, put it back in its intended spot if it moved, confirm that it is there or archive so the area can be freed up for a new cache.
You should not post a DNF log for example if you get to ground zero and realize that the cache is 40' up in a tree and you are not willing or able to climb to get the cache. It is not that you didn't find it, it is just that you were not willing to take the chance going to get it. If anything you would post a note in this situation not a DNF.
This cache should be a straight forward cache, with little option for camo or cover. If you can't find it then please post a DNF log so that I can check on it.