I may be completely wrong, of course, but: For me it seems so very clear, despite many arguments against it: The SP was seen on surveillance video, leisurely walking the hallways, doing some door opening or glass shredding. The video was of a longer duration than we have seen. LE said, the part of video, which wasn't made public, contained nothing important to see. After SP's strolling, breaking, looking into rooms, he disappeared from video surveillance. Because he once very clearly for several seconds looked in direction of one of the cameras, he knew, they were there and working, IMO. So I think, he also knew very well, when exactly to hide from surveillance, because the time drew near, when he expected Missy to arrive. Missy was a little late this morning, so he had to wait a little longer than he could have known. He stayed in whatever corner he was in and waited patiently (or not patiently). When Missy entered the building and was doing, what she did usually every morning under certain conditions (at church, very bad weather), he attacked her. What MB usually did at a church and before starting the session, was known by many: first one of her daughters, who sometimes accompanied her except this fateful morning, maybe her husband, maybe other relatives, the campers and who knows, who else. They would have known her routine and whether she opened the building for her campers to use the bathroom or for other reasons.
In short version: SP knew, Missy would arrive at a certain time and exactly in time he intentionally disappeared into a corner with no surveillance and waited for doing his job or mission. Why should it be a coincidence, that he just then wasn't seen any more, not even lurking around a corner or something? - As always: IMO MOO
PS: I'm thinking of someone, who is mentally very sick and now and then "needs" a thrill-kill. Years ago one day (when needing a large sum of money) he had the idea to hire himself out as a hitman. "Two birds with one stone" so-to-say.