How do mice get into your home return air vent?

For the past two winters, I have noticed that mice are nesting in one of my return air vents. I don't have central air conditioning so I don't know where the vent goes out? The mice can be heard in the basement up to my 2nd floor bedroom return air vent. Apparently, they can't or don't come inside the house because I have never seen not one dropping inside my house and I have set traps but nothing. How can I find out how they are getting in? And why in this area? I have sealed up all noticeable holes around the inside and outside of the house and low and behold they come back to that same area. This puzzles me to no end, they are just in that one area. There must be a way they get in from the outside. My house is 78 years old and I don't even know where my return air vents goes out to. Help?How do mice get into your home return air vent?
In many older homes return, and sometime supply, vents were ';panned'; which is basically saying that they used the floor joist or wall frames to make paths for the air to travel in.


Because these Pans are interconnected throughout your home, the entry point for the mice can be anywhere.


Today's codes while still allowing panning in certain circumstances, they require the panned bay to be ducted. This not only cuts down on your house guests but also some of the nasties that find the environment in an unlined panned joist a beautiful breeding ground.


Spring time comes around, consider having a professional duct cleaner work on your home. Many, using newer technology can actually send small remote cameras through the system identifying blockages, unwanted openings (Or were you intending on heating the attic with that duct?), and other potential issues.How do mice get into your home return air vent?
I don't know how they got in (they are tricky little things, like a magician), but maybe you should try those gizmos that emit a high frequency sound that drives the little critters away. You can probably find them at your local home improvement store. But make sure you read the precaution label because I don't think that they are safe for cats, rabbits, hamsters, Guinea pigs or other small pets.
Those little critters can get in anywhere.