In this case, the odds are pretty good. When MB walked in the entrance, she could have gone right down the hallway where the dutch door room is and the offices beyond that. Or she could have gone N up the other hallway, which is what she did.
SP, we know from a portion of the video that was timestamped, went S past Holy Grounds and the entrance, and then around to the dutch door room around 3:58. We don't know how much time he spent from there as he may have broken into the offices. But 20 mins is a long time, so I think the odds are good that he had explored down that hallway all that he could, then came back and we see him go thru a door to the sanctuary area (the last part that is shown to us). I think MPD intentionally didn't want us to see his last known location as the last section on the video - they didn't want us to make that connection. So the fact that the sanctuary area is last on video tells me that it wasn't really last.
I think by the time that 20 mins elapsed, SP had finished up in the sanctuary and had made his way back to the main hallway. It's a long hallway - a straight shot S to N. He was either at the alcove or in the bathroom, IMO. If not, he was heading back up the hallway. He wouldn't still be going into any rooms because he had already come from there and done that.
So chances are excellent that at 4:20 when she enters, he's either going to be right there near the entrance or the bathrooms or further up the hallway.