If the heat ex has a pin hole leak ,the central heating water will escape from the sealed /pressurised system ,internally within the boiler the escaping water will drain out through the condensate pipework.
With a condensing boiler ,the condensate ( slightly acidic condensed water) is a by product that is created when the boiler is in operation burning gas,and it drains out of the boiler via your grey pipe . so when the boiler has been cooled / isolated the boiler does not create any . Hence the water collected in the bag during your trial is either...
1. Central heating water escaping via the heat ex.
2. Water from another source completely that is entering the same waste pipework ,and clouding the issue.
3.rain water has got in your bag or someone put it there !!
It isn't always obviously apparent if the heat ex has a pinhole by a cursory visible check inside the boiler.
As high temperatures exist within the boiler ,and subsequent expansion and contraction ,the amount of water lost can vary and therefore the pressure differences.
Your kitchen rad thing is a red herring.
If you had a leak on the external pipework ,when you opened the boilers isolators ( at the end of your test ) the pressure gauge would have shown a pressure drop .
You should document the trials you have done and the results ,and pass them on to the attending engineer. There really is no substitute for an on site investigation /diagnosis , and the info you can provide to them will aid that.