How to unlock Anomaly maps?

I failed to unlock all the anomaly maps except one because I just can't find the necessary objects to open it. This is a very annoying mechanic.
I don't get it. Are the keys to unlocking (stones, etc.) always on all adjacent maps, or is it random?
Do you need a boss on the map? I decided that the stones only appear near the map boss. Is this true or not?
Do you need to kill all the monsters to see the key, or not?
This uncertainty is very annoying