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I think this is an STI only thing, but I'm not sure. While replacing my rear rotors this weekend, I found this mounted behind the brake caliper. Any ideas what the hell it is? I can't figure out what purpose it would serve!
I'm guessing some sort of vibration damper?

No clue. I looked in the Suby service system and can't find the part.

Any way to get a pic of it mounted in place?

Something for the parking brake perhaps?
Where / what was it mounted to?

It's mounted to the back of the caliper attachment point. I'll try to get a pic for you all. It weighs a pound or two and that donut looking thing is hard rubber.

Best I can do without taking the wheel off in the office parking lot As you can (kind of) see, the bracket is attached to the back of the caliper mounting bolts.

I've never seen anything like that on any WRXs / FXTs that I have worked on.
It is likely an STi thing and I'm still thinking it has something to do with the parking brake, but if there was no attachment other than the bolts pictured I can't imagine how that would work.

i wondered the exact same thing when i did my calipers too...

NVH attachment is my guess.

My 135 has a hard rubber chunk the size of a small fist attached to the side of the muffler to dampen vibrations. It's not near any of the mounting points of the exhaust.

^^That's the only thing that I can think. Some kind of NVH damper like J first said. I got nothing. I've looked at exploded views of the whole rear end multiple times and can't see that part anywhere.

Or it's a GPS tracker.