I've lived in this house for 20 years. The bathrooms (3) all have switches for fans, but the two upstairs ones do nothing when you flip the switch. Recently, while doing other work, we discovered that there are fans in place and a wire just sitting there not plugged in. On plugging each of them in, we discovered that when you flip the wall switch, you hear a fan sound and it appears to be working.
So why weren't they plugged in? Why did the previous people disconnect them? There are vents in the outside walls in roughly the right places, but perhaps the disconnected fan units in the bathrooms are not connected by any kind of venting or ducting to those outside vents. How can we tell? Is there something we could release into the air in the bathroom and watch for outside? If we go up into the attic crawlspace, what should we look for? If they aren't ducted, what are the consequences of using them - say in a -20C winter when someone is having a hot shower?