If the holes are neat and round about the size of a hole punch, it sounds
like the work of slugs which venture out do their dirty work primarily at
night when other self respecting pests are catching a few Z's. Try staying
up all night to nab them or simply try the saucer-of-beer approach (easier
on you and the environment plus helps keep thousands of brewing workers
employed).
Greg
grheck@macc.wisc.edu
University of Wisconsin