I say snag them. If they come back and ask about them, just tell them the truth. You thought they were gone for good, and the heaters would just go to the garbage and you thought you could make good use of them. If they want them back, then no big deal.
I'm a mover, and I'd say most everyone I move only leaves behind things like landscaping material, flammable chemicals that we can't transport, and building material (tiles that match the bathroom, wooden trim pieces, paint that matches rooms in the house, etc.). Otherwise people just throw it away. Unfortunately, my crew and I are usually asked to haul it to the curb or dumpster. But the good thing is some of the stuff is good. I've snagged boom boxes, tv's,(including the one I use in my living room) vacuum cleaners, the computer monitor I curently use, LP's, misc furniture, a refrigerator, you name it.