Okay, so, after a thorough examination, it appears that John-in-the-Wall isn't a 'he' but an 'it'. Several its, actually.
I was alone, everyone else was either out of town or working late, I was watching scary stuff on the telly and then just spur of the moment got up with a screw driver in hand and began pulling down vents. The intake vent was painted over, screws and all, so I had to pick the paint out just to find what type of screw driver to use. Turns out, all of them. There was a Phillips head screw, a slot head screw, and an Allen wrench head screw! Whoever put this vent back after removing it years and years and years ago used whatever was in their junk drawer to secure it. The only thing I didn't have was a flashlight when I removed the vent, and it was dark in there. So dark, in fact, that I quickly retreated from the enormous pile of long white fur that lay snuggled against the inside of the vent with the long-dead maggot larvae throughout. I called the sil and told him to bring home a good flashlight -- and a stick. Long story short, it was just a pile of dog hair, and the 'maggot larvae' were old rat poops. Not that that's good! But it's a smidge better than a dead animal and maggots in the vent. And the smell, it turns out, is just old dry rotted wood and drywall as the heater closet used to house a swamp cooler back in the day. A big, cumbersome, leaky swamp cooler. It's a matter of sweeping out the fallen rot, getting out the dog hair and rat poop, and spraying the whole thing with bleach water, but has anyone done it yet? No. Apparently I'm staying in an if-it-ain't-in-yer-face-it-don't-exist house.