While an intact male cat prowls through his neighborhood looking for a mate, that isn't the only thing that motivates his roaming behavior. This means that even after a cat is neutered, he may feel the urge to get out of the house and wander -- he may even be up to some of his old tricks while he's out.
Looking for Love
A neutered cat can no longer impregnate females, but that doesn't mean he won't go looking for a mate. Neutering significantly reduces your cat's interest in sex, and if he is neutered before he mates, it may eliminate it altogether. If your cat has had an opportunity to roam and mate before being neutered, though, he may have learned that it is pleasurable, and will seek out more mates even if he can't impregnate them.
According to the New York Times, a 2013 study reported that pet cats allowed to roam free are responsible for killing hundreds of millions of wild animals every year, including about 696 million birds. Your cat doesn't lose his animal instincts to hunt after being neutered, so when he sneaks out and disappears, he may be wandering through the neighborhood in search of his next kill, like a bird, mouse, shrew or rabbit.
Exercise and Curiosity
A cat who is accustomed to exercise is liable to maintain his lifestyle even after being neutered. Male cats have a natural tendency to explore, regardless of whether they're intact or not. While yours may or may not be motivated to find a mate, he may keep up his daily exercise just so that he can get a break from the house -- exploring nature, meeting other neighborhood animals, running and climbing may be ingrained in his daily routine.
A neutered cat doesn't typically retain his desire to mark territory, but according to VCA Animal Hospitals, about 15 percent of neutered cats still use urine to do so. This means that when your cat is sneaking out and exploring the neighborhood, he may be spraying urine -- which carries his identifying pheromones -- onto objects and places that he considers his territory. Even if he doesn't spray urine anymore, he may mark his territory my scratching it or vigorously rubbing it, to deposit the scent from his body's natural oils.