Loss of pressure

[Loofah]

I have a WB greenstar 30i in a pressured system feeding HW, ch and UFH. A while back the pressure started to drop and WB replaced the aav which seemed to sort it. No floods of water, maybe a small leak.
A year or so later the pressure started to drop again, WB duly came out and swapped the aav and the pressure was fine. Guess what happened next. Same again although this time the engineer was adamant the aav was not leaking but swapped it anyway and again the pressure was fine until yesterday when it started dropping again.
Given the above I'm relatively sure there's no system leak but have no idea where the water can be going as I top up the system when the pressure drops. This is not a chronic pressure drop at this point but I'd like it resolved as system or boiler and it to be permanently sorted.
What if anything can I check?

 

[kidgreen61]

Somewhere in your system will be a pressure relief valve, most likely near the expansion vessel.
Have a good look whether that PRV is letting water past.
It may be that your expansion vessel needs recharging, I believe there's a how to in the FAQs on this site.

 

[Loofah]

Thank you, I'll have a search for that FAQ.
I can't see any obvious prv leakage but have attached a few pics of the valve, expansion vessel and tank in case it helps

 

[kidgreen61]

That white exp vess is for your unvented cylinder, there will be another one somewhere, assuming your boiler is a heat only boiler, and not a system boiler.

 

[Loofah]

There doesn't seem to be another vessel anywhere.
I topped up the water over the Christmas break and for the first time noticed a drip! Having taken off some lagging there's a very dodgy looking joint, clearly this is a leak even if not the leak. The draw off also seems to be slowly dripping so that can be swapped at the same time.
How I've never noticed this is beyond me as it right in front of me when refilling. The lack of a puddle is explained as the drip goes straight down and into a void at the edge of the floor...
I'll get this fixed but my biggest inexplicable thing is the inconsistency. I guess corrosion at the leak point can explain that to a degree but why did swapping out an aav stop the leaks?