With Heather missing, Stiles was in full on detective mode trying to figure out what happened to her. The only thing Stiles could think of was the alpha pack. It sort of made sense since teenagers were the best candidates for turning. Why they would need a beta wasn't really clear to him but it was the only lead he had.
The only clue they had to find the alpha pack was somewhere in Isaac's head. He had found them, he saw something big and the alphas took away his memories of the situation. Peter had managed to find out bits and pieces by getting into Isaac's head a little but more effort was needed to get everything.
More effort apparently involved a bathtub filled with ice water.
( Bath time for Isaac )[Taken from Teen Wolf 3x02 "Chaos Rising". NFB, NFI but OOC is fine]