Our downstairs toilet seems to half-empty itself after flushing. I am
sure it never did this before. You can flush it, it seems normal, and
half an hour later the level in the bowl has dropped by a few
centimetres.

What could be going on? There's nowhere for it to go other than escaping
by climbing over the U-bend, and no sign of any leak. I am pretty sure
that no-one in the household is siphoning it away or drinking it.

Daniele