Piccadilly line train at parsons green

I don't think Picadilly Line (1973) stock is banned from that line, (it has certainly been seen at High Street Ken and Ealing Broadway on occasion) but I can't think of any reason why it would go down that branch. According to CULG tube stock is allowed as far as Putney Bridge, but "1973 stock units contain an auxiliary static converter which can interfere with signalling equipment, and this must be isolated before the train can run on any line other than those specially immunized"

But I suspect what you may have seen was the track recording train (TRT)
www.flickr.com/photos/24315579@N08/10840316014/in/photostream/
Tube size cars, and from the side looks very like a 3-car 1973 stock unit - indeed the middle car is an actual 1973 stock car.