How would maggots get inside a wheelie bin?

i put my wheelie bin out for the bin men on monday morning....ive just gone out to collect it now and i noticed it was outside someone elses house...i wheeled it back to mine, opened it to put rubbish in and saw a load of maggots in the bottom of the bin yuk yuk yuk?! now how would they get there...inside a wheelie bin with a lid on....surely someone would have to have out them there?