VW tensioners move even when cold because the action of the engine causes tension to go up and down as the engine moves and warms up. It's supposed to be set cold so don't worry about the warm position. If you touch the tensioner, make sure the tab on the back is in the slot. I've seen a defective or damaged tensioner where the tab was bent. I don't know if it was from prior installation or some other problem but it could have let go, so you may just want to quickly inspect it before reuse.