It's leaking down the thread of the taps.
edit - having fully read your post perhaps not, but I had this many times when people were rinsing the bath with the shower head.
They are now owned by a different company, and the boiler is completely different, yet they still allow the plastic flue.
The evomax is proved to be a reasonable boiler.
There shouldn't be any circulation through the fne tank unless badly designed.
Magnetic filters are always recommended.
If you do seal don't forget the filling loop.
If it ain't broke and all that
There is nothing wrong with open vent, I've never like inverted cold feeds especially as your system is dirty.
Sealing it is more work than its worth, there is no real benefit but a risk or two.