While I am in the camp that BT may have had eyes on him prior to even exiting his hotel room and making his way to the conference.Given that the shooter was on the phone just prior to the shooting, it was postulated that a watcher/accomplice might have been involved. Someone who informed the shooter that Thompson was on his way.
What if the shooter was pretending to be on the phone since, in the video, he has someone walking so close to him?
Plus, the phone was dropped in the alley and recovered so maybe the shooter knew it would recovered and he was most likely on security cameras 'using' it and was hoping that police would be thrown off his scent by trying to track down and unlock the phone giving him time to get away?
I'm just spit balling here.