The mice that I thought were gone from my greenhouse have merely been dormant.
Last night I covered everything in the greenhouse with bubble wrap because it was a clear and cloud-free night. No clouds this time of year means frost. When I went out this morning to uncover everything and to check that the lobelia seedlings survived without heat (they had), I discovered evidence that the mice had been in the newly sewn seeds. They dug up and ate every sweet pea seed! There was scattered seed compost over my staging and the hollow shells of sweet pea seeds scattered across the top of the disturbed compost. I think they got every single seed.
Now I have to go and buy new sweet pea seeds and re-sow them. This time however, I am going to keep the sweet peas in the house until they are well sprouted so that they will be less of a temptation for those rotten mice.
I've also set a mousetrap on the greenhouse staging. If the mice come back, SNAP!