A very cost effective solution
In my opinion the delivery guy will only come back with the motorbike if they also have something to take back from the employer. I mean the delivery man can deposit something valuable before heading off to deliver the goods. Something that's more important to them than an iPhone. Home keys or some money maybe (yeah, they get more money out of selling the iPhone...), ID cards. Secondly, they don't have to know what they are delivering. Use undisguisable boxes, it can contain az phone, jewelry or a pack of cards from the looks of it. Depending on what they deposited it's a gamble to steal the goods. During the screening of a new employee ask them what is their most valuable property (even sentimental ones), so you'll know what to ask for as a deposit. If they don't have anything valuable I don't think a tracking device will stop them to get away with the goods. It would be the first thing to get rid of.
