I've watched the Arrin Stoner video. The footage was just that little bit clearer. To be honest, to me it's looking more like he actually did put whatever was in his hand in her purse, and I'm inclined to think it was his phone. I'm not saying I'm sure of that, so I agree with the statement that it's inconclusive.
Ed talked about a theory around this on Unfound Live on 1 Dec. If you want to get to the Brain Shaffer section of that stream, go to the 42 minute mark. Whatever you think of the theory, I think it's worth a listen.
Remember how Brian was going to walk the girls to their car, and for some reason that didn't happen? Just suppose Brian did want to drop the phone in a purse that would be on the move, with his motive being to look like he's going one way when he's really going another. Offering to walk them to the car could have been his way of getting an opportunity to plant the phone. When the opportunity presented itself sooner, and the job was done, he would have had no need to follow through with walking them out. And as far as his overt interest in Brightan that night goes, that would have just been about picking someone he thought was an easy enough target for this.