plumbing
Our house has an attached greenhouse; under the sink there is a device connected to the cold water supply that then goes into the slab. There is a floor drain in the slab, so it's possible it connects there. What is the device called, what is its purpose?
I would call it a slip x spigot brass drain street ell. Can't say I've seen one until now. More common is the double slip brass drain ell. Hard to say from a photo, but looks like a short horizontal straight piece could be inserted if you had to use a double slip ell. Perhaps the trap U could be rotated away a bit to buy a little extra distance for this.
The only time I ran into something similar it was due to a failed anti-sweat mixing valve on a toilet that allowed back-flow through the hot water pipes. That would also explain in your case why the kitchen sink doesn't seem to be effected by the issue.
About the only way I can think of to test this theory (without opening walls) would be if you can cut your cold water off to the bathrooms without cutting the supply to the hot water heater. In the unlikely event that you can do that, your toilets would still be able to fill from the hot water line when flushed.
Best Answer
Suspicion coupled with an image search says yes, it's (almost certainly) a trap primer for the floor drain. Makes sure the floor drain trap does not dry out when disused and allow sewer gas to vent into the space.