Thank you! We actually have pretty bad water pressure so this all makes sense. I'm pretty sure the valve has been working as designed. Mystery solved.Yes that's called a "Dual Check Valve with Intermediate Atmospheric Vent". When pressure is higher on the boiler side than on the water feed side, it dumps some water, rather than going back into the potable water feed.
It may only be the result of lower pressure on the feed side due to high use but you could check to see if the pressure climbs above the typical 15 to 20 psi(assuming that's your typical-?). If it climbs high, it could be the result of a water logged expansion tank. But if that was the case it would like be releasing more water.
They do sell repair kits if it's just a matter of a weakening unit. I'd probably live with it, once I checked out the possible causes.