At the plant we called that “midnight dropoff”
Back in the 80s there was a pier off City Island, Bronx, NY that had a manhole near the end. Unmarked tanker trucks carrying whatever waste would park next to it and drop a hose through the manhole to off load into Long Island Sound. Worked great until one driver didn’t replace the cover and the SHTF. Far as I know, no one was ever arrested or charged.
Around the same time I would drive the LIE near the Fair Grounds around 3AM. I would often see a dripping, gunk covered tanker truck highballing from the Island toward the city. One look and you just knew it was bad news.