Mice will infest any location that has food and while a garage is not generally a food storage area it is often used for storage of other things, especially for a barbecue. Garbage can also be stored in a garage and if it isn’t locked and sealed then that also becomes a food source. There may be garages used for the storage of food as well. While canned and jarred food is safe, things like bags of rice and grain and especially dog food are not. Depending on what you store in your garage mice may be tempted to live there. They may also make a nest in the garage simply to get away from the cold or the heat. Mice will nest just about anywhere. No human-built structure with four walls and a roof is safe. Barns, sheds, garages and even your home are all places they will hide. Even if there is no food in a house they will use it as a secure place to breed and then venture outside to forage. This is why even an abandoned house with no food in it can have mice. If there is food, no matter how old, they will eat it.
Finding a mouse nest is difficult. Even if your garage is just made of a single layer of plywood they hide well and in secure areas that are difficult to access. They will live in a disused chest of draws, inside the walls of a house or garage if there is a space for them and inside of tool boxes and even barbecues. They are very dangerous creatures because they defecate in their nests and where they eat. Their feces can cause serious stomach pain and upset. If you have mice in a garage or shed, or even in your home, finding them is not as important as treating them. While many people will go straight for snap traps, capture traps, glue traps, or domestic grade rat and mouse poison, all of these traps have serious downsides. Snap traps and glue traps are gruesome. The mouse often won’t die from a snap trap and will still be alive and wriggling. Glue traps are not humane if multiple mice are trapped in one they will cannibalize each other. Domestic poison runs the risk of a mouse dying inside the walls of your house and capture traps are only effective if you take the mouse very far away from your home to release it. Otherwise, they will find their way back and reenter the house. They most often enter through an open door, they can fit through an opening as small as a quarter if your garage door does not meet the ground in some places they can use that for ingress into the structure. The same goes for a back door left open or any entryway that is not fully secure. Space under a door or on the sides will often be enough space for them to squeeze through. They can also enter houses through intentional openings. Wall vents, weep vents in brick houses and foundation gaps can all allow a mouse to enter your home, garage or shed.