Update:

I gave up with this after emptying a full bath produced no results.

Some months later her neighbour had a pinhole leak in a mains pressure copper pipe under the floor, fortunately it was above a car drive through so no damage downstairs.

When this leak was fixed her problem disappeared.

The damaged pipe was close to the area of the mystery puddle, maybe a couple of meters and at a slightly higher level.

Too much of a coincidence not to be connected.

All that remains unexplained is why the leak was intermittent but I think the case is closed.

Thank you all for your help.