That's what I thought too, but if you look again, there appears to be a second register towards the left where UG (unknown guy) is paying. I really just think he happened to be at the staore at the same time- he comes walking from the left side of the store in the video and up to the counter so he was there before the two came in. imo
That's not the original video that was first shown, but a subsequent one now "cropped" to eliminate the unknown guy's entrance just before MB came in. In the original video, UG came in, stopped several paces inside with the door still in view, and stared up at something to his left. Next scene is MB entering with DJ close behind, but iirc no sign of UG. UG reappears in the back of the pic as MB shown at the counter grabbing/dropping cigars, with DJ standing behind him looking on. UG sees the theft in progress and moseys out of the store, holding the door open for no discernible "innocent" reason imo.