The City engineer came out today and poured bright green dye down the drain. It did not show up in the sanitary or storm sewers. He still believes it was sewage though, even when I showed him the gravel/mud in the drain with a flashlight. His theory is that the drain trap was cast iron and rotted/rusted away over the years, but the rest of the drain is still under there somewhere and connected to the sewer. Another drain in our exterior basement stairwell was found to be sewer connected. He doesn’t think a plumber would have connected one but not the other.