Usually, around this time of year, the beans are too high to see groundhogs. I'm often reduced to waiting on the perimeter of the field just before dusk, hoping to nail the ones that live in the fence rows or edges of woods nearby as they return to their holes after eating. Have to work fast, as they don't lolly-gag there in the (usual) grass strip of "No-Hogs' Land'" between the two areas.
Can't recall having ever seen a woodchuck out after dark, but maybe it's an evolving way to avoid predators - although many of those around here are more active at night.