Okay, big confusion here...
Two hideout planets for the Founders!
1) One seen in "The Search" and destroyed in "The Die is Cast"; well hidden, probably heavily defended nevertheless, allowed Odo and Kira to land because they knew Odo would be aboard. After all, they had just raided his ship!
2) Another first seen in "Broken Link", last seen in "What You Leave Behind"; again well hidden, probably a bit farther away from the wormhole than the previous one, no doubt heavily guarded. Only invited guests ever reached the orbit, let alone the surface. Starfleet probably still doesn't know the whereabouts.
For all we know, the Founders swap hideouts every century or so. Hideout #1 must have been there for quite some time because Odo remembers it; or perhaps he was preprogrammed with the visual memory of the nebula back when the Founders were actually hiding elsewhere and only planning on moving to Omarion. Hard to tell. But the Founders could be expected to have multiple backup plans and locations, originally for hiding from their own Solid subjects, later useful in hiding from external enemies. And the heavy guard around each hideout would probably still be fairly limited in size and extremely trustworthy and secretive - no shore leaves ever allowed...
Timo Saloniemi