Thanks for the replies.
Stainless tank. I have the problem with a full or empty tank.
Cooking. I don't think I would create that much moisture, maybe if I emptied the pans on the floor.
I've checked all the plumbing and there are no visible signs of leaks anywhere.
It's definitely not weather related.
Last year I had a period of about three months with a dry bilge. If I had a leak from outside I would assume it would be constant.
I have checked everything when the pump is running. The only place I can't get to is under the shower tray, but the water is clean, no evidence of soap or bodily detritus 😳