If it’s tar it would be thick, tacky and stick to your fingers. I think gas would probably evaporate rather quickly unless the leak is significant. Touch it, smell it, everyone knows what gas smells like.
That leaves brake fluid, a thin oily substance with not much smell, but oily when rubbed between your fingers.
Up north here brake fluid leaks happen when lines rust through. You said it happens in heat so I’m assuming you live more toward the south and aren’t likely to have rust issues.
Did you hit anything that my have gotten thrown up against the underside of your car and damaged a fuel or brake line? Rock, wood, dead body? (Animal)