Mechanical question, not-on-fire edition

This afternoon I saw something rolling out of the hood of a parked sedan (something nondescript, we’ll call it a 15-year-old Taurus). I walked into the emission to see whether it smelled like anything or was just steam, and sure enough it smelled like burning rubber.

A guy was sitting in the driver seat so I asked if it was on fire, but apparently it wasn’t and this wasn’t unusual for him. I walked away, but only after letting him know I had a fire extinguisher if he needed it.

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So: what’s the most likely cause of a steady stream of rubber-smelling steam/smoke from the engine bay? It was coming out by the windshield, not the grille.