Plumbing – have a vent stack if none of the fixtures are vented

A shower drain needs a trap, so check under the cap on the left to see if there's a trap or a straight drain. If it's straight, then your assumptions are likely right that it's a toilet and the other one is a clean-out (fix that cap, you don't want sewer gases getting it). If you don't have a shower drain here, then I'm guessing it's only roughed in for a half bath.

I don't imagine the 2" would be anything other than a vanity drain.

With the "vent," I'm left wondering if it's really a drain since I see a clean-out installed on it. I'd recommend tracing that line up the walls, as best you can, to verify it really is a vent.

As for where the horizontal line is going to the left, perhaps a washer dryer, or perhaps the plumber screwed up the T and it should be pointing to the right for the sink. It's hard to tell from here.

Most homes will merge the vent pipes as they go up the walls so that you only have one vent exiting the roof. All vents need to slope upwards, so the guest house couldn't run the vent underground to use the vent stack in your home.

That said, if you notice that drain lines are gurgling after the water goes down (indicating suction) or are slow to drain but have no clogs (indicating it's trying to overcome the air pressure in the lines) then there may be a blockage in the vent or it may not be properly installed.

Note that for some situations, you can also have an air admittance valve to provide a one way vent (fresh air can get sucked in, but sewer gases cannot escape), which allows you to vent plumbing in a kitchen island or other location that cannot go up and out the roof.

For the odd smell, it's difficult to diagnose over the internet, but it could be a dry trap, a failed wax seal under the toilet, or a dead animal in an exhaust vent or inside a wall.