This is a Challenge Cache and IS located at the posted coordinates. In order to claim a find you must have met the terms of the challenge. You may sign the log at any time and post a note on the cache page. Any "found it" logs that don't qualify will be deleted. There is a challenge checker you can run to see if you qualify.
A year ago I found a challenge cache that required me to find a mystery cache on each of 200 calendar days. I only had 65 at the time so I've done a few since then and found I really enjoy them. In fact, I've placed a few of my own since then.
Your challenge for this cache is to have found 100 mystery caches. You don't need to find them on different calendar days. If you find them all on the same day, that would work too. If you don't have them yet, this is a good challenge to work for.
Cathy'sClown has used Project-GC Challenge Checker to see if they qualified for this challenge and they did. I have found 322 caches of type Unknown Cache.