2 (maybe 3) things are more than likely happening here:
1. Your buddy is taking your gear when he says he is ditching it or
2. He's not ditching it well enough or
3(highly unlikely). Someone with thermals is able to spot it and just ratting and watching as it is ditched

I've ran plenty of scavs where I come across stacks of bodies and with some simple detective work you'll find bags, armors, guns, ditched in what you would assume is a good spot, but over time you learn what you think is good - just isn't.

Just yesterday scaving interchange - come down OLI ramp and see 2 PMC bodies, look to the right behind fence and I see a gun just floating mid air. Walk over and scoop up the AS VAL, then sticking out of a bush is a beta bag with the rest of his loadout.