Hot water cylinder (direct heating), loosing heat and pressure during the day
Sounds like a leak, near the kitchen sink that allows hot water to drain from the heater towards to sink. 

One way to confirm (apart from seeing the leak effect) would be to turn off the isolation valve at the heater for the day, while away, and checking by how much the temperature drops during the day.

If it doesn't drop nearly the same, then you know there's a leak and the cooling was due to cold water replacing hot water in the tank.