I am still a 'newbie' so someone else may have a different opinion. But I log a DNF if I looked for it and didn't find it. I figure if I didn't find it and enough people log a DNF then the owner will go check on it. Maybe it's not there and if no one logs a DNF then the owner will never know.

I haven't logged a DNF when I arrived and the GPS batteries died and I didn't get a chance to look really well. Or if I didn't get a good chance to look and was going to come back the next day. Then when I didn't find it, I logged the DNF or if I did find it, I usually mention that I looked the previous day and was unsuccessful but finally found it.

I think some cachers may think that a DNF is a bad thing so they don't want to log it. Maybe they feel it makes them look bad?? Not sure.