empty pond

came home from a week away to find the lilly pond all but empty. Bit
mystified as there's no sign of a leak anywhere and the walls of the pond
beyond the plastic liner don't feel soggy. And the fish pond level had
dropped less than an inch (we've had a couple of dry weeks).

Same thing happened last year, lilly pond merrily ponding along, then
suddenly in dry weather emptied itself. I filled it up and it stayed ok
until this week's little performance. Is there something large out there
with a great thirst? Or am I missing something obvious?

cheers

Jon