Impeller Locknut
 

Hey Guys, apologies if this is posted in the wrong part of the forum, but I'm new to the board.
I've registered specifically to ask about an issue I've had, and I'm hoping that one of the experts on here can help.
How likely would it be for an impeller lock nut to work its way loose on a new turbo? I had the unit replaced on my car 4 months ago,after it blew on the motorway, and paid for a new turbo, hoses etc, but a couple of weeks ago I noticed another noise coming from that general area. I removed the hose between the turbo and the air filter, and inside it, as well as a load of oil, was the impeller lock nut from the spindle.
How likely would it be that this would have just worked it's way loose if the turbo was new?
I took it back to the same garage, who then replaced the first unit with one that was clearly marked as a reconditioned turbo unit. And then within 50 miles the entire engine seems to have given up. The noise it's making is sounds like the camshaft is doing its best to force its way out of the block, and I've got oil in the engine coolant, as well as seemingly everywhere else.
Is it at all likely that the problem with the turbo may have, directly or indirectly led to the other problems I've now experienced? Obviously, I have no idea when the lock nut came off, so could it be that I drove like that for some time during the time between the 2 turbos being replaced?
The car was only a Focus TDCI, that now looks destined for the scrap heap, and I'm no petrol head, so any changes in performance would not really have been that obvious to me unless it was drastic.
Would appreciate any comments (helpful) that anyone would care to leave.
Also, apologies if I appear to ramble. It's 4.45am, and I've not slept for the third night in a row!

Extremely unlikely if the turbo came from a reputable source.