(Set up is 2 Toilets on one waste pipe and exterior venting through
roof.)

Noticed that after flushing loos that water level gradually drops
about 2cm over a couple of mins - Looks like siphon effect and I
understand this can be caused by a semi block in either waste pipe or
roof vent) or windy weather. However doesn't appear to be a block in
waste pipe, weather conditions have been variable and I cannot see a
block in the top of the vent. (Issue is same in both Loos and prob
appears to be recent).

One additional point - I can top up the level in the pan at any time
after flushing and it will still drop approx 2cm no matter how little
or how slowly I add the water. Suggests to me no siphon effect.

Am I missing something obvious...anybody have any ideas please?