Having had to use a fire extinguisher inside our shop, I concur that it looks like extinguisher residue. We had to hose down a police car we were lettering when an errant welding spark caught a blanket on fire.
Burning blanket ended up on the hood of the police car, and the car was quickly sprayed with the extinguisher. It leaves a yellowish fine powder. Washes right off with no ill effects.
