For a multitude of reasons, using ductwork to sneak around a building is a bad idea. But if you are committed to infiltrating a building through the ductwork, we recommend using a kitchen exhaust duct (and make sure to enter when the kitchen is closed, so you don’t end up like this ninja).
These systems are designed for you to be able to get inside of them. Grease builds up in these ducts as part of the normal cooking process and needs to be regularly cleaned out. The fan on the roof is hinged to allow access for cleaning–or a ninja. The ducts also have access panels for cleaning. These features make it possible to actually get into and out of the ductwork.
Kitchen exhaust ducts also are free from any obstructions in the ductwork. Normally, turning vanes, dampers, and filters would prevent you from traveling very far. But these features are not allowed in kitchen exhaust systems because they would trap grease. You would actually be able to travel from the exhaust fan on the roof to an access panel or the kitchen hood without getting blocked.
When the kitchen is in use, the air coming out of the exhaust fan would be hot. In this comic, the ninja should have noticed the warm air and known to not use the duct at that moment. But that would make for a much less interesting end.
Mark Robison, PE