Top up tank has very hot water - Pulsa coil 111

Any thoughts as to why the top up tank (manually filled) on a Pulsa coil 111 has very hot water in it.

Everything seems ok, both status LED lights are green and the overheat is not popping out. The reservoir water level is ok and the float switch is working, so it just seems odd that the reservoir tank water is very hot. (not scolding but very hot). Only the overnight economy seven (off-peak) immersion heater element is used, the on-peak immersion heater is always switched off and never used.

Any ideas as to what causes this or what fault testing could be done to identify the problem would be gratefully appreciated.

I usually check the water level in the header tank once a month..but left it a lot longer and the tank was almost empty and hence no hot water!. Luckily when I refilled the tank to the max mark I got hot water again. I was surprised I did not have to press a reset button to get hot water again...just wondering how the pulsacoil managed to produce hot water again just by refilling the header tank?.ALSO just in case the same thing happens again and it does need a reset after I refill the header tank WHERE is the reset button please..is it just one button??..I have Re checked the water level and it is constant so I don't think there are any leaks..thanks