Crap. I emptied our tub this morning, and forgot to turn it off. I can't be sure (because we had a powercut while we were away at the weekend, resetting the tub clock), but I suspect it ran a 2hr filter cycle towards the end of emptying. Unless it's clever enough to turn itself off when it senses no water, I suspect it'll have tried to run the circulation pump for an hour or more.... how screwed am I? It's an Antigua 2 from https://betterlivingoutdoors.com/product/hot-tub/5-person-antigua-2-hot-tub/
Fingers crossed... won't be able to test it until tomorrow I think :-(