The short answer (read this first)
Havenwise's hot water control doesn't just heat the water right when it's needed. It finds the cheapest time to heat, based on the outside temperature, when the water is needed and your energy tariff. Water is a great store of heat, so it stays warm until it's needed.
You can read more about how Havenwise heats hot water here.
The longer answer
Sometimes people notice their hot water heating even when no hot water is scheduled - for instance, when they're using holiday mode. This isn't a result of any commands that Havenwise is sending, but instead something else at work:
You have a solar diverter
Many people have a solar diverter which they have forgotten about, or haven't touched since the previous summer. However, Havenwise can't control your solar diverter.
If you have solar panels and notice your hot water heating more on sunny days, it's worth checking the settings on your solar diverter to ensure it is only active when you want it to be.
You have a legionella cycle scheduled on the controller
If you still have a legionella cycle scheduled in the controller, Havenwise is unable to stop this running. This can lead to inefficient hot water cycles and wasted energy. You can read more about how to set up a legionella cycle on the Havenwise app here.