Yup...I've watched them eat frogs, salamanders, crickets, slugs and a host of other smallish invertebrates.
If she keeps seeing them early in the day it a spot that gets great early sunlight they may just like the spot to warm themselves in the mornings. Garter snakes, and many other snakes like to lie out in the sun early in the day to help get them moving since they are cold-blooded. They have a fairly limited individual range and they often nest very close to the areas you see them in, so there is probably some prime snake-estate in the area of that raised bed.
