The lazy plumber never removed the plugs after the inspection! That's why the joints weren't glued up, so he could retrieve his test plugs. Rain, or melting snow does come down the pipe, and it has no place to go...it could build up, and without the connection being glued, it could seep out.
SO, remove the plugs, and do what you need, then glue it up. Keep in mind that if you put too many elbows in there, if a bird's nest, chipmonk, squirrel, etc. decides to invade the pipe, you'll never get it cleaned out.