I worked in Roads and Drainage division of Public Works for a few years. The Sewer Inspectors found all kinds of good stuff. Manny diamond rings and other jewelry.
I have known two women who lost 20,000+ diamond rings down drains and never got them back they searched the clean out traps.
Gold and other dense metals will naturally collect in the low velocity pockets of a running water system you don’t even need metal detectors just a basic understanding of flow regimes and gravity.
This holds true in nature too as the places to look for gold in placer mining is well placer deposits of sediment the gold will be the last sediments washed out of a flume.
I would expect in a city the size of NYC and in the wealthy Manhattan borough jewelry would be packed into the low velocity pockets just outside a hotels outfall into the combined see sewage system. So yeah where they’re looking is precisely where any jewelry that went down a hotel drain would naturally end up its not a bad idea for recovery of diamond rings which soap loosens up and boom down the drain it goes. Happens VERY often.