Mystery pressure loss

Looking for ideas as to where a pressure loss is coming from in my heating system, gas boiler.
Didnt notice any problems, everything running ok, and I would occasionally look at the boiler as there is a hook in the kitchen cabinet for tea towels, so Id throw an eye over the pressure gauge anytime I put the teatowel on its hook.
I noticed one evening recently that the heating system boiler gauge pressure was at zero,
I wondered how it could have dropped and the only conclusions were,
1. A leak somewhere inside 2. An airlock 3.The Pressure relief valve operated.



As Im painting a room, I removed the head of a thermostatic rad valve in that room so I could paint up to the pipe and close to behind the rad without getting paint on the thermostatic control head, just the screw on part with the adjustment was removed. This was earlier in the same day I noticed the problem.



I thought I might have release an airlock as that room never seemed to heat up fully (wasnt being used but heating was on), I checked and I did have to re-pressurise the system from the fill loop (very occasionally Id have to do this maybe once a year max), the odd occasion Id put some inhibitor in too, but Id only ever have to bleed the rads a bit.

In this instance I had to bleed some rads downstairs and all of them upstairs,
As I thought job done, turned off the fill loop,

yesterday again, found the same thing, pressure to zero, re-pressurised, but checked rads, nothing needed bleeding, pressure came up to 1bar quick, I had the manual open lever set on the valve that directs heat to rads and tank in all instances, pressure was lower than 1bar going to bed,

obviously I wanted to monitor it,


Checked this morning, pressure at zero, in each case I went and checked to see if it looked like the pressure relief valve that has a pipe out the back wall behind the boiler appeared wet, difficult to tell on the first 2 occasions as it was dark and it had been raining, but there appears to be a bit of a web in and around the pipe and it didnt look wet or like there was an water in there, and this morning the same. Im thinking of pushing some kitchen roll up there, not completely block it, just twist some up the copper pipe to see if it gets wet?

On the first top up, I was cautious Id see a leak show up inside the house when I refilled it, but none (and hopefully that stays that way), and the heating was working after each top up and even before I topped up, although the first time there was definite sound of sloshing and gurgling like the pump was pushing an air/water mix, but once pressurised, I thought it'd be ok.
Im fairly sure the expansion vessel was only replaced no more than 2 years ago.
My hot water from the tank is not showing any signs of dirty looking water and heats up fully when pressurised, and I think it has done so even when the pressure was slow/at zero.

I wrote off the idea of it being the boiler gauge as there is a gauge in the hotpress which has followed exactly what the boiler gauge is doing.


But I dont know where the pressure loss, which must be a leak is going somewhere?


The only thing I can think , of is have both stats (hot water & heating set high) call for heating on both channels on the programmer but turn the boiler off (the pump will run, but the boiler wont fire up), I had already set all the thermostatic rad valves as high as they would go when bleeding previously, now Im thinking take off all the heads when doing so?



Im at a bit of a loss? Any ideas?