Problems Starting My Own Cache Series

Over the last few nights I have been laying out caches for my own series of caches around Alconbury. I am going to call it the Alconbury Amble.

I submitted them on the geocaching website and the reviewer, Red Duster, came back and said "Your cache is placed too close to the final stage of an unknown cache (GC16ZV2 which is within 55 feet)"

This confused me, there was nothing on the map. What does unknown mean? If it's unknown that how does he know about it?

It turns out that it is a Mystery type cache, therefore the published coordinates are not necessarily where the cache is placed. Not only that but it's a premium member only cache which means I wouldn't even see the published coordinates.

So Red Duster has given me a 30 day free trial membership which will allow me to see the 'puzzle' cache and hopefully solve it, therefore letting me know where the final coordinates are that I need to avoid!