Recently, I noticed a couple of garter snakes, one larger than the other, lurking in places where mice or chipmunks are likely to pass.

One favors a spot by a window well under the edge of the cover. Coming out the door, I can see its head, which draws back as I approach. Yesterday, a chipmunk was drinking from the water bowl on the porch. When it saw me, it dashed along the walk, towards the snake. The snake eased out, ready to strike, and the chipmunk made a heroic leap, clearing it by a good eight inches.

We live in a mountain valley that's very cold, with no venomous snakes. So we don't have to be nervous about their presence. In fact, I'm glad to see them as they are voracious predators of slugs, bugs, caterpillars, and mice, which can be a plague here, gnawing on seedlings.
Scanning the back yard, I figured out where the snakes den: there's a gap in the stonework on a retaining wall by the greenhouse. This morning, I saw a snake ease out. I wonder if they're a mated pair and we'll see a brood of teeny snakes.

Their choice of a den is quite shrewd: the rock wall gets afternoon sun, so it stays warm into the night. Being near the thermal mass of the insulated greenhouse, it's also warmer than the earth in general.