I'm moving in a week today after only 3 months living at my current address. (NOT by choice!) The kitties have twigged what's going on as they remember the big boxes from last time. One of the little darlings has taken a shit in a huge box of my clothes. I've had to completely unpack, remove faecal matter, and wash everything at 90.
Do I have another 6 days of this to look forward to?
Your kitties are clearly smart. if I were you I'd give them plenty of fun and games with toys and boxes. Pooing outside normal area is usually territorial. Make sure they still feel like they own your space. even if it's full of boxes. good luck