I don'tknow if this is where I need to be but here goes. We have a freon type smell in the living room of our house. I think it is coming from a vent that is underneath a 12' window. We've had this smell for about 2 years or so. We've changed the filters about every other month or so. There's nothing around the air conditioning. This is the only room that has the smell.

I have used freon(Dupont trade mark) refrigerant for many years and never smelled it. Now the oil mist that is in the refrigerant can be smelled.

What does the vent do that you feel the odor is coming from?

Do you have a gas furnace that vents to the outside near the window?

I know for sure that freon smells like propane (freon has no scent). The manufacturers have to make it smell like something in order to control the substance because of its high toxicity. I don't know the number for the environmental protection agency in the States, but I think you should be calling them to report it.

There are requirement for vent locations and under a window isn't one of them. It may have to be relocated.
Can you take a pic and post using "go advanced" feature of the forum.