You can see when the hot water has been turned on or off (be that manually, or via a schedule) by looking at the activity log inside the app
If your heating system has a single timer that controls both heating and hot water (sometimes encountered in older systems) you may hear the boiler operating in the night even if the system is in Slumber Mode. This is probably because it is heating the Hot Water.
To avoid this situation instead of setting your timer to “Always On” you could set it to “On Once” and set the times to something like off at Midnight, on at 06:00. This would mean that Cosy is only working over 18 hours (06:00 – 23:59) rather than a full 24 hrs. Please note, this is not a fault of Cosy, it is due to a limitation in the heating system in that the Central Heating and Hot Water circuits are controlled by a single timer.