OK, as far as the gas cans - yes. Anything that could have any decomposing cells on it that are (yuck) rotting. The decomposing cells give off that gas that the dogs are trained on. We actually got real pieces of cadaver - legally through Tallahassee of course - medical science donated cadavers - to work with. We would put them in a test tube (tiny tiny piece like pinky nail size that was just flat skin), cork it up and bury it - deep! The could find it every time - my job was moving it without anyone knowing where I put it. It was amazing to watch. Clothes with the "smell", gas cans, even people's feet that have walked through the actual stuff and tracked it. Wonder if they hit inside the house anywhere? Actually laundry detergent does wonders for killing this scent.
As far as false hits - if a handler is lazy and doesn't work the dog and make them live up to their potential every time they practice, the dog gets lazy too. Dogs are smart. "If I bark and sit, I get to play with my toy and get lots of pets". They all try it during training. They don't get disciplined in anyway, just ignored and no reward. Usually they figure out it won't work but if it worked even once for them in training, they might try it if they are hot, thirsty, lazy or just not into it. I seriously doubt this with the LE/PD. They pay these handlers to work these dogs routinely - done on the clock even. The dogs can sense the excitement and apprehension and know this is serious business. They usually work better in an actual search than in training! They love it - this is what they love to do and live to do! False hits do happen but a second dog verifying should shut up that argument quickly. LE uses a second dog on really important cases too so that the "false hit" defense has no oomph behind it...
Sorry so long - trying to get the whole info out there..