Boiler pressure goes back up after draining

We have had the classic 'low/zero pressure, top up, back to low/zero some hours later issue. We had British Gas come and do the necessary, including repressurising the expansion valve and all seemed well.

Next day, the pressure was just a whisker under red so I drained a little out via the rad nearest the boiler. Being in a hurry, I left it at that. This weekend, I resolved to drain it down to a more reasonable pressure so attached a hose to the drain point and waited until my glamorous assistant shouted "Stop!" However, the pressure gauge then immediately went back to the same point.

This would point to the filling loop leaking (it's a Worcester Bosch boiler so has a permanent, fixed loop not a chromed, flexible loop) but Mr BG checked that and I know both taps are turned off.

Of course, I will call BG to attend, as we have a service contract, but I'm curious to know what might be happening, so that I can make adjustments in the future without having to wait for them to respind

 

The boiler is less than 5 years old, no 'hot then cold water issues and no banging.

It all works as it should: I just don't like the pressure being so much higher than it should be.

Thanks for the responses so far!