Enviro Grow moisture reading steadily rising

I’ve provisioned my Enviro Grow, and have on moisture sensor in a plant pot at this point, while I fiddle with a web front end for it.

I’m seeing an odd set of readings, though. When first plugged in, at 10am yesterday morning, it was reading 23.67. It rose gradually throughout yesterday, and by 8:30pm had reached 26.67.

After being switched off overnight, this morning first thing it read 25.17 - and now it’s up to 27.33.

No water has been added to the pot during this time, and it’s on a reasonably warm window ledge - so if anything I’d expect the readings to be going down.

Is this normal behaviour?