Strange burning smell after driving, but only sometimes, what could it be?

[rogelioburch]

This has been bugging me for about two weeks now. Every so often after I park and get out, I catch this faint burning smell near the front of the car, sort of acrid, kind of like hot plastic or rubber. It's not constant, some days nothing, other days it's there. No smoke, no warning lights, temperature is normal, and the car drives completely fine otherwise. I checked under the bonnet and couldn't see anything obvious, no melted wires, no leaks I could spot. My first thought was clutch, but it's an automatic, so that's out. Then I wondered about the brakes dragging, or maybe something with the AC compressor, or even just oil dripping onto something hot. I found a Helpful guide to what a burning smell from your car means while googling around and it listed a few possibilities I hadn't considered, which made me realise I should probably get this looked at properly instead of sniffing around and guessing. My question is how do you narrow down the source when it's intermittent, and is there anything I can safely check myself before booking it in? Also is there any smell that's a definite "stop driving immediately" signal versus one that can wait?