Nasty smell from left rear wheel-well. Brakes?

spearfish25

So I got home today and noticed a nasty smell that seems to localize to the left rear wheel-well. I haven't smelled anything like this before, but my suspicion is the brake/pad. It's a sour, pungent, burnt smell but much more sour, twang your nostril smell than I've ever noticed from burning brakes. I've been running Carbotech XP10s front and XP8s rear even while on the street to ease the constant swapping. The other three 'corners' don't smell at all.

Any thoughts if this is truly the brake pad or what else it could be? How would I tell if that left rear caliper is dragging the pad and not fully releasing?

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2013 Cadillac V-Wagon, RIP Z