Gravity sink drains can’t have anti siphon loops, but they usually have valves to close underway. Can we assume you removed the barbed fitting on the grey flex hose, from the white hose? Was the other end of the grey hose ever connected to anything? I’d have guessed it was an anti siphon loop for a washing machine or dishwasher or some other appliance that pumps its discharge out. Was an appliance removed? Sometimes the breaker panel will identify old stuff that’s no longer aboard.

 

Ahh you’re a wise one. I think there used to be a washing machine there that’s since been removed. We’re currently changing the thru-hull and fittings so I guess I can skip the T-piece and remove that hose entirely then? Or better to keep it in anyway you think?

 

Looks like there was something mounted where the grey hose was cut off. No doubt this was a flux capacitor modulator relief valve vent. Just kidding. I have no idea what that was for.

 

The double hose clamps make me think that it must have been important, whatever it was for.