The Plumber Knows He Forgot Something. He Just Can’t Remember What.
When plumbing a new house, plumbers must pressure-test the waste pipes to ensure that the system doesn’t leak. Leaks might allow waste water or sewer gas to enter the house. So, when the drain piping is complete, the plumber inserts a test plug and fills the drain system with water all the way to the top of the vents on the roof. This allows the plumbing inspector to check for leaks. When this test is done, the plumber removes the test plug, seals the drainpipe, and goes onto the next job. On this house the plumber forgot to seal the pipe.
The test plug is that long black tube below the pipe. It had been inflated inside the pipe for the pressure test and was subsequently removed. Perhaps the plumber then went to lunch and forgot to come back and cap the drain pipe. In any case, someone should install a cap now, before a backup causes sewage to flood the crawlspace.