A-holes like him would try and use Probable Cause by you admitting you have an extra mag as justification that their lives are in danger to search your vehicle.
There just no reason for people to be like that.
Which is why I wouldnt ever say anything about it. If asked directly "Do you have any extra magazines, ammo, things like that?" I would st,ate "Yes, I have a box of ammo and mag in the glove box."
He can have the satisfaction of taking that box, which is missing 33 rounds, and that extra mag, and I will just ask him to put the cartridges in the box, so they dont get lost. If he asks me to get out of my car for some strange reason, I will deal with that as the situation evolves. I would have dug myself a hole, but whatever.