Which bit of my bike just fell off??

Hi all,
New bike with XT throughout, putting wheel back on and this fell off – possibly the XT rear mech? I can’t seem to work out where it came from! It’s a curved rubber cover for something or other…
Thanks in advance!
Andrew

Looks like chain guide?

Some rubber cover over chainstay end? Can’t really see the detail as it is too dark (the benefit of black rubber!)…

There’s not much to it – only about 2cm long, curved outer edge so is covering something with a curved shape, inside there’s enough space to wrap around something quite thin and there is a little ridge suggesting there’s a notch in something that it fits into.

Goes here

Looks like a bumper:

https://www.sjscycles.co.uk/gear-spares/shimano-deore-xt-rdm8100-bumper-y3fw50000/

Edit: as above!

Thanks! STW is the best, I’m not sure I would ever have worked that out…falls off a bit easily btw…!

I’d have never had got that.