Oh my goodness!
I think I may have the evidence of The Seat Creep I've been testing for (feels like I've spotted the Loch Ness Monster)...
As you can see, the right hand side of the tape has moved considerably (the two RH sides were level, until this morning).
What's interesting is that I'd noticed no movement at all ever since I put the tape on, over a week ago. But yesterday, when I was leaning across the car, doing the vinyl roof wrap in my garage, I kept accidentally locking/unlocking the car because the key was in my pocket.
I DID NOT get into the car. I DID NOT accidentally touch the seat button (I'm in the UK, my car is RHD, so the seat button is next to the gear shift, not on the outside). I finished the roof wrap late at night, so I left the car in the garage overnight, unlocked, with the windows down.
Got in the car this morning, to reverse out of the garage, and noticed it felt different. Looked down at my tape - and you can see what I saw.
Ooooer!
Now, I'm as sceptical as the next man - and I can hardly believe my eyes, but the evidence is there, I think?
Any ideas how it's happened? Should I try replicating the movement by locking/unlocking the car several times again?