If someone had told them you/someone were wandering around with a gun, the AR would've been there faster than you can say "throw down your weapon"
I think they're spinning you a yarn, although I don't see why.They go onto private property and remove unattended articles? It's a farm ffs, there's allkinds unattended. They should have checked with the farmer.
Is it worth considering ringing plod before you go out - should save any problems in future.