Thanks for your kind replies (I like the black hole theory!) - my language was poor in that regard, so to clarify - After checking again this morning, I think water is beading/collecting on the inside of the cover and dripping out via the flaps, down the sides of the tub, in an usual quantity. I searched around the entire perimeter of the tub and inside the maintenance console an there is no sign of a leak anywhere, nor any standing or frozen water anywhere but down the sides of the tub. This morning there were huge icicles down two sides of the tub that weren't there last night, and as I check the lip of the cover, they are definitely wet and the origin of the icicles. Given the volume that I see it wouldn't be impossible to lose what I did over the week we were gone.
If :
I continue to find that there is no leak;
The air dials are closed (I never have the air jets on when the tub is not in use, and we only occasionally use them when we are in, so they are definitely off)
The cover (a flat, basic version) seems to be at least a big part of the issue. I've not read about anyone else having these drips down the sides of the tub in cold weather, so perhaps it doesn't fit right after all. Strange, because it's the cover that came with the tub, and it *seems* tight around the edges. Even though it's tight, the flaps are still wet.
Anyway, thanks for conversing and offering ideas about this. It will feel better to make a financial commitment for a new cover or blanket or whatever at least having explored possibilities, so I appreciate this very much.