If it's a seized brake caliper, you should hear some squealing or grinding or squeaking noise. My slider bolt seized on my rear brake and made a lot of noise.

If the smell is kind of sweet smelling, almost like cotton candy, it could be antifreeze. Do you see any green residue in the engine bay or any puddles under your car?

As for your sparkplugs, I'd say to take a look at the wires. Are any of them melted?

Does the smell occur if you recirculate the air that's inside the car? If not, I'd suggest looking around the vents under the hood to see if there's any debris or anything. Also, next time your car smells bad, pull over and try sniffing around the engine bay to see if you can find where the smell is coming from. Good luck!