Strange behaviour from unvented cylinder

[essayer]

I have been trying to track down a leak in my boiler pipework for some time. Leak sealer fixed it for a year, but now it's returned and I need to fix it. (Megaflo, oil boiler, separate DHW + CH loops).

In the meantime, I top up once a day back to 1 bar via the filling loop.

A few times recently, I have heard water continuing to run from the incoming water feed for about 10s after I close the filling loop valve.

It stops if I turn off the mains - I took off the filling loop and it's not leaking from there - so the water must be going into the tank

What would cause this?

[smallpaul]

Not an expert but we've had problems with expansion vessel faulty and another time needing recharged.