I've been doing a little bit of leg work for you all
I managed to get talking to an eye witness on Twitter and I now have a pretty good idea of the chain of events. DISCLAIMER: whilst this is probably accurate, I have made a few guesses as to what has happened going off what I've been told. If this is not accurate then I apologise, as I was just using evidence available to me at the time.
From what I understand, a black rubber cover off of one of the upstop/sidewheels (I was unable to identify but from the witness' words I think its the side ones) flew off when it went over the second airtime hill leading into the cobra roll. Apparently it came from the back of the train, so I assume it was one of the sidestop wheels on the final car, specifically on the train's left side. In their words it was a "black ring".
These are the exact photos they used when describing the part. And this is where they said the part landed:
Soon after this was seen the ride was E-stopped (possibly using on the newly installed side boxes?). A member of staff near the ride area was also seen on the phone soon after the part "flew off".
Very soon after the part flew off, someone picked up the part and gave it in to Towers staff, where I was promptly taken away. Nottoo many people saw to get pics, so that's get for Towers' media rep
As Mako said, at 4:19 the ride reopened, so it's not a long term issue. One of the trains has been taken off to the storage track at the side, presumably to have a service later. This is definitely not deserving of the coverage its been getting for certain though.
EDIT: I'm afraid I can't give the source's identity as I feel it would be a breach of their privacy