The trains have been in the engineering workshop, which is located down the service road behind where Survival Games is (behind Swarm). They've been there a couple of weeks.
They are then suitably packed to be moved to the maintenance building for the ride, which as we can see is happening now. I was down Monks Walk earlier but didn't see it, so they're being moved quickly.
Another person I wholeheartedly trust with any 'information' regarding Thorpe Park. Thanks Josh, that makes a lot of sense.
Just one thing though, I don't think they were ever all fully unpacked as such, the wrap they're in has Mack branding on it. I think just unloaded from a container / lorry and stored in there until they're moved to their own maintenance shed. It was confirmed in Jack's video that, before testing can start, Mack will be onsite checking over the trains in the Hyperia shed once they're moved there. (I could be wrong though, I don't know for sure, just basing that speculation off the branded wrap and what was said in Jack's video.)
I do wonder though, if maybe they actually have filmed a reveal video already, using just the front cars, and what Swat heard has some truth to it... I guess time will tell. Reveal video should come in a few days either way, if not not tomorrow.