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Is the problem a continuing issue? AC and heating vents are for the most part semi enclosed to entry except from inside of the house. Unless the ducts have been breached in the attic, it is unlikely that you would have any animal in the system.
Do you have natural gas heating? Natural gas has an odor added to it to warn users if it is leaking. That would have access to the vent system and although unlikely, worth having it checked if the problem has continued with no other identified cause.
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