Where do vanishing prims go, and do they use sim resources?

I am sure we all have had this happen. I just unlinked 2 older link sets, and one vanished. I have rebooted my sim, relogged, cleared my history. I don't know the sim prim count before this happened so I can't say if this is eating my resources. And it was only 6 prims. But where do these vanishing prims go? And how do you make sure that they are really gone and not just ghost prims that are being counted by the sim?