I agree with hawbsl. You could (and probably do) have a large population of small slugs hiding in your soil as well as larger snails and slugs hiding in the leafy parts of your garden during the day. To combat them without pesticides, you can use a combination of approaches: 1. go out into the garden after dark and collect the little critters, making sure you look in the areas surrounding your veg and flower patches, and discard them (chuck them over your garden fence or dispatch them in a glass of cheap lager); and 2. if available in your area, you can buy nematodes that will infect and kill your slug (but not snail) population (this is the brand available in the UK: http://www.nemaslug.org.uk/) There are other non-toxic methods (there are organic slug and snail pellets available), but I've found these two approaches to be the most effective.
I've also found that coarse sand can also act as a barrier to snails and slugs, particularly in the greenhouse or other moderately-protected areas.
Additionally, I grow all my lettuces in planters and ring the planters with copper tape which, in my experience, is great at keeping snails off tender plants e.g http://www.organiccatalogue.com/catalog/product_info.php?products_id=1756. This time last year, my potted hostas were nearly decimated by slugs, but this year there's not a hole in sight! BTW, I've found that snails and slugs climb right over the copper rings that sit directly on the soil and they ignore sunken beer traps, but maybe I have fussy slugs.