I have the same exact situation. My closet happens to be next to a bathroom. The bathroom vent fan happens to be in a space where the ceiling was lowered about 12 inches for the a/c system and vents. There is extra space and both bathrooms are there, so each bathroom has a vent fan with exhaust to outside. I took out their old crappy, noisy fan and put the vent cover back up for show. There was only a single rafter I needed to go over. I bought the matching 3" tubing, an adaptor and ran my newly extended line over the rafter and into my grow room's newly cut hole to match the tube. I bought a new bathroom exhaust fan, attached it to the wall near the ceiling and attached my tubing. Check my journal to see the pics I just put up today of the inside of my closet. It all slowly came to me, kinda like creeper from my teenage days. LOL. Even if you had to vent into the empty space, there is usually an eave to vent the heat build up in the space separating you from the outdoors so heat doesn't transfer into your apartment. But without the built-in existing like in my apartment, you risk the neighbors smelling it. Trust me, I don't know what my neighbors are doing, but sometimes it REEKS of paint thinner and I know it's not anyhting in my apartment.