The smell emitted by the plant is irritating for mice and they try to avoid them. Poo from bobcats, snakes, and cats are also effective mice deterrent because of their distinct smell. That smell scares mice away due to their natural fear on their predators. Pet stores have products that contain this unique smell which you can buy to spread over your house to keep mice away.
1) A myth exists that a cucumber smell comes from rattlesnakes. 2) The Tongue Mountain Range is known for harboring rattlesnakes. 3) The only place I have ever smelled random cucumber is the Tongue Mountain Range. Which leads me to believe the tiny minority, that rattlesnakes do in fact give off a cucumber smell.
Chances are a plant, not a snake, is producing a cucumber-like smell, and the presence of a snake is a coincidence. There are actually several fern species that have a smell similar to a cucumber, and one would expect ferns on a hiking trail near a river. Snakes like to take shelter under ferns, and of course, snakes have to drink water, too, so they tend to be found near bodies of water, especially in hot weather.