Mystery 120v plug under hood

Hi everyone,

New (used) Prius owner but long time and proud Echo owner

I was relieved that the car came with what appeared to be a block heater, as it had a plug tie wrapped under the hood that is identical to my echo block heater plugs. I rigged it up with an extension cord and routed it out through the upper grill.

After plugging it in on a few cold mornings I didn't notice any difference in the warm up. I confirmed this by monitoring ECT after startup... plugging in the car makes no difference.

Turns out the plug cord doesn't actually run to the back of the engine where the heater element plugs in. It coils up behing the passenger headlight. After passing into a cord-wrap it is routed back toward the fuse box on the passenger side, then it appears to route into the cabin.

Not a block heater. Does anyone know what this plug is?!