A couple in Hamburg, Germany had the same problem. The mystery was solved when they found a disc in the mailbox that showed 12 short videos, apparently taken by one of their neighbors, showing another of their neighbors "watering" the landscaping.
The quote that prompted me to link?
“It makes me feel sick when I think how often I ran my fingers over the leaves and wondered where the liquid came from,” said Mrs Freibel.
One neighbor is a human sprinkler, another is a voyeur/videographer.
I believe I'd move.