My group completely avoided Sandor's room and went straight to Ilnerik. Ilnerik is now dead, and the PCs have left Walcort. Ideas on what should be done with Sandor now? Should I leave him in Walcourt just in case the group decides to go back and look around? I'm thinking of holding him until close to the end now.
That is actually a good question.
Sandor the Strange (Mother of Flies, p. 43) has only a limited loyalty to the Drovenge siblings. Remember, he is only serving the Council of Thieves as a means of serving his true masters.
He will be able to observe the aftermath of the Player's assault. He has to make a judgement. Does he still think that the Drovenge siblings are going to win?
Consider also that in part six, the Council itself doesn't have a lot of the arcane at its disposal other than the Hellknight Sorceress, so Sandor would make a good stand in for one of the many Council captains that permeate that module. The Drovenges could easily fund the resources to intice Sandor to stay a little longer. Sandor seems practical, so granting him magical trinkets for future services seems a likely scenario. Consider also how dangerous the Council of Thieves and the Drovenges are; since Sandor is known by them, he might not be able to just 'walk away' for fear of the eventual night when an assassin's blade finds him.
I would not have him encountered at Walcourt; without the distaction of allies and the tight quarters of the hideout, he's going to get one-rounded barring some extreme luck.
Consider also that in part six, the Council itself doesn't have a lot of the arcane at its disposal other than the Hellknight Sorceress, so Sandor would make a good stand in for one of the many Council captains that permeate that module.
This is a smart way to recycle this character. It also gives him some back up to prevent him from being "one punched" by the heroes.