What has been eating my pots?

Having been away for nearly 3 weeks, we returned home this afternoon to find 14 plastic pots in the garden which have been munched by something. The pots contained some rare poppy plants which I have grown from seed from a plant I bought last year from a chap who persuaded a gardener at the RHS to give him some seeds. I think I will be able to salvage 10 of the plants but 4 of them are gone. I would love to know what the culprit is,