I didn't go round two days ago, it was at the end of January but it's still not that long for him to adjust if they've just put his tray back down. At the end of the day, he's got to relearn indoor litter training in a certain area again.
He's such a nice cat, and to be honest his owners do seem to love him but I think his behaviour is confusing them so much that they're at a loss. Has to be said though, he's an RSPCA cat so legally aren't they supposed to return him? (I hope not, but something makes me think that's the case.)