Have someone jump up and down in the bed, maybe you can see something then. Does the liquid feel oily?
Fix a flat (the stuff in cans) used to be highly flammable-explosive to an extent. The propellant used in the cans was at one time PROPANE, or a mix of it. I don't know that they're using now. I want to say CO2 but I am not positive.I think the warning you have in mind is from the spray cans of tire sealant. The propellant in the can get in the tire and may cause grief. But that propellant has probably been changed by govt. regulations by now!
Antifreeze is what I suspect too.....radiator, recovery bottle with overflow tube all in the front. While I had tire off started it in the garage and let it run 15-20 minutes to get the temperature up and nothing...didn't see any leaks. Scratching my head!