Purge is capricious indeed.

I sheltered my treasures in one of the lemurian big room relics with one natural entrance.
First time, failed.
Second time, I removed 1 fence that surrounds the treasure, failed again.
Third time, nothing changed, the purge miraculously triggered.
Forth time, nothing changed, same as above.
Fifth time, they spawn right in front of the entrance, almost door-to-door with my base.