In vanilla, there is no NPC pool, released slaves are erased and never seen again.
In Aric's Mod, there is an NPC pool, but the function for releasing slaves appears to be missing, so the game will probably generate an error and do nothing. Edit: oops checked the raw mod files not the final files. Arics's mod should have the same result, unless he did something crazy to add them to the NPC pool.
For freed slaves, they go about their merry lives in the infinite void. For escaped slaves, I did something crazy and added them to the NPC pool. Eventually(TM) I would like to see a “willingly returned” feature where the freed slave has a chance to return asking to be in your service again but until then if you want them to be gone, freeing them should do the trick.
It should be in the change-logs somewhere in the NPC's expanded release. It should work on named NPCs exactly like it does on non-unique NPCs. Instead of being removed (vanilla) on escape, they are added into the NPC pool and "moved" to the closest outskirts to the Mansion (ie: wimbornoutskirts by default). Searching in that area SHOULD turn them up at some point that day, but if you can't find or can't recapture them for any reason then they have a high chance to move to one of the other nearest locations (depending on their movement AI). Theoretically, they should move a step further from you but they are still out there and will be until captured by you or the authorities (ie: town events).
