Hot printed circuit boards, of course. Probably a blow dryer on max temperature for 10 minutes against an old PCIe card would give you an idea of what that smells like. (Do this outside, obviously.)

I would go for that tarry smoky smell you get from a heap of bbq briquettes just as they are set on fire.

Ask an LLM chat, of course.

Ozone?

Melted plastic