I just happened to be looking at Uxbridge on Google Maps and, as you
do, zoomed in to see what had been parked in the sidings.

I thought there were three Metropolitans and a Piccadilly (from
length) till I looked closer and realised that what I thought was
Piccadilly was, in fact, a train of 1983 stock.

I don't know when the photo was taken, but I wonder if it's the one
that Wikipedia says was at South Harrow? Any reason why it would be
moving about?