Hot water tank is feeding Cold water tank

[Wingspan38]

For some reason, Hot Water is finding its way into the Cold Water tank, irrespective of whether the C/H boiler is on or off.
All of the system's various cocks and valves have been systematically closed and re-opened in sequence, in an attempt to isolate the source of the transfer, but so far to no avail.
What is difficult to understand is that after the C/H boiler has closed down for the night, I have run the Cold Water tank down to below the ball valve level, so that the tank will then refill to ball valve level (nominally 3" below the overflow pipe). This ensures that the tank level is correct and that no water or C/Heating activity will take place overnight. However, in the morning, before the C/Heating boiler cuts in, the water level in the Cold Water tank has risen high enough for it to exit via the overflow and the temperature of the water in the Cold Water tank has now become warm.
Any suggestions?

 

[HERTS P&D]

Turn off your water mains before going to bed and see if it stops.

I'm going for mains water 'passing' through a mixer tap/shower.

Andy

 

[Wingspan38]

Thanks Andy,
I will give it a try, see what happens and let you know.
Ralph

 

[Wingspan38]

Hello Andy,
Your suggestion about shutting off the incoming main water supply sounded good, so I did as you suggested with to me, a very surprising result. After the C/Heating had shut down and all other services declared unusable, I lowered the level of the water in the Cold Water tank and allowed it to refill back up to ball cock level. When this was reached I closed the incoming main supply stop cock. No services (including loo flushing) were used after that time throughout the night.
Prior to the C/Heating cutting in, I went to check the level of the tank to see if it had, as hoped, maintained its level. To my surprise, it was empty.
I checked the house to make sure that no taps, internal, or external, had been left open and that there was no flooding anywhere inside, or outside the property. With everything in order, I am now asking myself where has the entire contents of the Cold Water tank disappeared to, without leaving a trace of its departure? Any thoughts.