Really hard to tell who's the culprit unless you catch him in the act, Greg.
Go out after dark with a flashlight, or very early in the morning after rain or watering has happened. Look on the under sides of the leaves with the flashlight. Look carefully under the bottom-most leaves too. If slugs or snails are hiding under there they wouldn't necessarily leave a trail.
Could be slugs, or snails or caterpillars or bugs.