My mother has an allotment, and to one side of the shed (the dark, wet, nasty side) there is a water tank type thing with a tarpaulin over it. Said tarpaulin is held down with bricks. Earlier today, my mother took the tarpaulin off, and found a large stash of plum stones in there, all with a hole 'drilled' through them, roughly in the centre. There is a plum tree on the allotment, so it's a fair assumption that they came from that. Whatever put the stones there had to be able to either:

• Squeeze underneath the tarpaulin
• Move the bricks, take the tarpaulin off, put the stones in the tank then cover it up again and put the bricks back.

The explanations we've thought of so far are:

• Aliens (unlikely, but we should cover every angle)
• Birds
• Squirrels
• A disease or creature that eats through the plum stone, then the plum is eaten by a bird or squirrel and the stone dumped in the tank.

Does anyone know of anything that may have done this, or had anything similar happen to them? I would appreciate any help.

NOTE: I haven't seen one of the plum stones yet, but should soon. Until then, I may have difficulty answering some questions about them.

Gall wasps lay their eggs inside all different plant material, acorns etc.

The growing larvae then burrows its way out leaving a neat little hole.