Hiding a geocache: beware of mystery caches

After having hidden a few geocaches over past 18 months, I faced a slight problem with the latest one I hid this weekend . I had selected a place on a small hill, from where the view over the lake below was quite nice. A few other caches were listed in the neighborhood, but they were something like 2 km far from planned cache. So, on Sunday, all the family went there, and we placed the geocache, after a one hour walk.

Back to home, I filled out the submission form, and waited for it to be reviewed. But when I received the reviewer notice, I was quite surprised: the reviewer was requesting me to move our cache, as there was another one at less than 120 m. I checked with him, and rapidly understood the origin of his request: the existing nearby geocaches were mystery caches. Their coordinates on the map are not their coordinates in the real world! And it is not possible to get those real coordinates without first solving a puzzle associated to each cache...

So, THE question: how to be sure that I will not hide a geocache too close to an existing mystery cache, if I don't solve associated puzzle before? Reading Geocache Guidelines gave me the answer: a mystery cache must not be more than 3 km away from location posted on the geocache web site. Consequently, my next caches will always be placed more than 3 km away from any existing mystery cache...

And I have to move my cache  A good reason for another hike, next weekend