Is my ferret depressed or possibly sick?

I am glad to hear that you had 4 ferrets total and not just the 2 bonded ones. Ferrets become very bonded to each other and their owner. If Patches was bonded with the one you lost and not the other so much, it will be especially hard for him. I have had that problem before as well. As long as Patches has had his cage mates for a while I wouldn't be too worried. He will probably be depressed for a while. Just give him lots of attention and keep an eye on him.