Over Pressure Valve, aka: Expansion Valve.
As you can see, those are ultimately interconnected by hoses ending up in the drip tray. My theory was that if some residual water from the OPV could’ve been in the circuit and, due to the heat caused by the anti-vac valve or expansion valve, it could be boiling the water and pushing the water down to the drip tray. Now that I see this image, in the way they are connected, it’s almost impossible.
It does happen with the Elizabeth though, and that’s why some people say that the machine gurgles and hisses but there’s no sign of a leak.