Toilet water missing

[ekmdgrf]

I've noticed recently that every now an then, the level of water in the toilet bowl has dropped, sometimes fairly dramatically. It always refills if flushed. It seems to be too rapid to be a leak and only happens occasionally. I would surmise that it has to do with pressure moving the water through the U bend. It's not a massive issue. I'm really just asking to satisfy my curiosity: what is likely happening here?

 

[Harry Bloomfield]

I've noticed recently that every now an then, the level of water in the toilet bowl has dropped, sometimes fairly dramatically. It always refills if flushed. It seems to be too rapid to be a leak and only happens occasionally. I would surmise that it has to do with pressure moving the water through the U bend. It's not a massive issue. I'm really just asking to satisfy my curiosity: what is likely happening here?

AAV, air admittance valve - stuck/sticking, It should be at the highest point, sometimes in the loft.

 

[MNW67]

It occasionally happens in our house when there is a strong wind from a particular direction. I think in our case it must be causing some sort of suction in the soil pipe.

 

[HERTS P&D]

Check your manholes for blockages.

Andy