Very high pressure on water heater tank (boiler) when not in use

Hello! I have a 114 litre, 2 x 3kW electric water heater. I live alone so only use the top heating element to heat the water on a timer most mornings, and otherwise it's turned off. I am frequently away from home for a full day or two. Above the 114 litre tank is a smaller red "golf ball" tank. On the hot water pipe leading out of the 114 litre tank to my shower (not the pipe between tank and "golf ball") is a pressure gauge. This pressure gauge reading creeps up above 4bar (the maximum the dial can show), well above the red line at 2 bar. This happens when I'm away from home for many hours, whether or not the boiler is turned on. When I run the hot water, this dial drifts back down to just below 2 bar. The water heater works fine, there's no 'strange noises or 'explosion' of water when I turn the hot water on while the pressure gauge is above 4bar. The 114 litre tank says "maximum pressure 150 psi, test 300psi" which translates to 10 & 20 bar, well above the 2bar red line maximum on my pressure gauge. QUESTION: should I worry about this pressure gauge showing 4bar+ pressure (double the 2bar red line)? Again, whenever I turn the hot water tap on, the gauge drifts back down to just below 2bar. Thank you!