A friend had rats in her shed but it was during the winter months. They had both a way in and another route out. They ate through a plastic jar of bird food. They did access under the shed. We have put our shed on slabs and also compost bins for that reason.
Not noticed anything in the shed yet luckily, but thanks for mentioning as I'll have to keep an eye out. Hadn't even occured to me! I've given the garden a good clean to make sure there's nothing to attract them and keeping my bins in the shed as there's typically cat litter in there which I'm hoping the scent will scare any rodents off!
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If there are enough of them under the shed they could dislodge gravel pushing it out.