We know that SP was first seen on camera at approx. 3:50 a.m. and that MB was first seen on camera at approx. 4:18 a.m. It is true that no one found MB until approx. 5:00 a.m. (but keep in mind a camper was waiting in his or her car in the parking lot starting around 4:35 a.m.). And its true we dont know the exact time the perp left the building. However, logically, the perp got out asap and didnt wait around for anyone else to come in (assuming this was a targeted killing, which I think it was). I suppose one could entertain the idea that MB was there awhile setting up while perp hid, but again, that doesnt seem logical considering perp would increase chances of being caught or seen leaving. LEs indications are that MB went in and the crime happened immediately thereafter otherwise, LE probably would have stated that she was setting up for a while before the crime happened. She went in and walked toward the location of perp and was never seen on camera after that
I think if she had been there 30 minutes setting up, LE would say, we see her on camera for several minutes setting up, etc. or going back and forth to her car. But they dont say that, so I think LE believes she went in and was killed very soon after entering. See LE press conference at below link.
https://youtu.be/RW5edymTlw0
Adding LE affidavit stating MB entered building and walked toward location of suspect and never seen again on camera.
https://assets.documentcloud.org/documents/2822749/BEVERS-SW16-060.pdf