What clue points to the culprit?
It was Bowles who signaled the driver to pull into the hotel lot. Knowing the location of their stop, he then called the backup car lagging a few minutes behind. He knew that Sims had parked in the area behind the hotel, and while the van was being parked, he finished the phone call.
Sims had a key to the cab and for the lock on the back doors of the van. But the two minutes in which he was alone at the van were not sufficient time to break the lock and also the case that held the Audubons. He only had the two minutes Keene mentioned to get his luggage, re-lock the back and front of the van, then walk into the reception area of the hotel. Keene could not have alerted the backup car while in the cab with Sims, and Keene had left his cell phone in the cab. So he could not have done it while walking across the parked van to join Bowles.
It was only the art expert, Bowles, who had the time alone with his cell phone, during the brief period when the van was being parked, to inform the backup car of the van’s exact location in the rear parking lot. The backup car did not have to be nearby. It could have been as far behind as five miles and have received Bowles’ signal on the cell phone permitting it to arrive within five minutes.