I can't speak for Cod, but it sounds like he works in construction so I will speak my two bits.
we see it a good bit on job sites where Union sub contractors will jack up the work done by non-union sub contractors simply because they aren't union.
Some times it just petty stealing loose equipment, or small pieces of hardware, stuff that happens on a job site anyway, but only seems to happen to non-union subs when unions are on site.
the middle is a kinda vicious non-compliance. when a trade comes in after others have worked there is almost always going to be some overlap. framers frame the walls, and plumbers have to come in and cut the walls for pipe. good plumbers are going to minimize that interference, and are responsible for making any needed repairs. Unions will go out of their way to rip up the wood framing as much as possible if the framers aren't union. and typically with the contracts on union work it is much more difficult to get unions to repair the work or give fair value for the fix, that it is a non-union sub.
the worst is straight up violence and stealing equipment. multiple thousands of dollars worth of stuff stolen stuff, not just the loose hammers. anything that isn't nailed down they will take if it isn't "union approved", which typically means made by a union. they will tell the GC they had to remove it because it was in the way or a danger or whatever clear BS they can come up with. and that's if you can prove it was them taking it, but again when its only non-union work getting hit, its pretty clear. it will even get to the point that the unions will go after non-union made work trucks, slashing tires, smashing windows, B&E. usually when this happens it turns to straight up fights because its hard to hide. if the cops get involved the union lawyers swarm the place and bury it legalese bs, and most non-union subs don't have the money to fight them, even if they can prove it.
it is very much the mafia. you don't park near them, don't eat near them, don't question them etc etc.