Garden slugs are attracted to dark, moist areas where they can hide. They'll find the grapefruit rinds when they're out at night looking for snacks and crawl inside, resting there during the sunny daylight hours.
You can use a variety of fruit rinds to catch garden slugs, including orange rinds, lemon rinds and lime rinds, halved cantaloupe or honeydew melons from which you have scooped out the fruit, or winter squash rinds again halved and hollowed out. See also Molasses Garden Pest Control Spray.