Think about this for a minute...not saying it is what happened at all...but just one in tons of scenarios which could be possible:
Bio mom finds a way to talk to Haleigh and tells her she will come pick her up in the middle of the night if she won't tell anyone. She gets Haleigh to unlock the other door (reason the kitchen light was on) and go willingly with her. Bio mom sets the cement block up to stage a stranger abduction before getting Haleigh after waiting in the woods on the right time to get her. Suppose the things they found are not related to Haleigh, but to someone else (cig butts, water bottles, etc.).