This is why I think JI came home, DB said something to JI after they saw BL wasn't around, something specific pertaining to SB (perhaps based on a comment that SB said to her while outside or a vibe she got from SB while outside), the first place JI goes to is SB's door? Now, why is that. We're starting to find out now (via RR) that it's not like DB and SB were life long friends. Now, you can look at that in two ways (depending on where you stand). If you believe they are guilty, you can say that DB needed to find someone to pin something on, SB is the first person that pops in her head (because that's the last person she was talking too) and tells JI this so JI goes to SB looking for BL. If you believe they are innocent, DB realized BL is gone and now something that SB told her makes her think SB took the child.
Consider this. Everyone has been criticizing (and most of it is warranted) DB about her behavior that night. What if as the night went on, SB started making comments to her about that as well, like 'you should be checking on your kids' or 'do you think it's a good idea for you to be out all hours here drinking'. SB has also been drinking, and usually as you drink, more stuff comes out of your mouth that you wouldn't normally say. Now, I know this would be hypocrisy, since SB was doing the same thing with her own daughter, but perhaps the comments were more pointed toward BL (she's a baby, you should be checking on her more). DB goes inside, goes to sleep. JI comes home, BL is missing, it goes like this:
DB: Go check SB's house
JI: Why?
DB: Well, she was giving me crap earlier about checking on BL
JI: You think she would take her out of here?
DB: Who knows what going on in her head, she was going on and on about cheating on her husband, giving me crap about BL, etc, etc, etc.
JI goes to SB's house and knocks on the door. SB answers the door, says I got no kid. Perhaps JI doesn't believe her first and asks again, SB tells him again BL isn't there, she would never take a baby, etc.
JI calls LE. Elapsed time = 15 minutes