Boiler backflow preventer leaking
This can happen if the pressure difference goes the other way, like if the water feed is shut off. Some common models like the Watts 9D-M3 have rebuild kits available, but they cost almost as much as a replacement valve, which is $30-35 in the US. (I just checked that at one of the big box stores here)

So the answer is replace it, though a plastic cup after 3 weeks is pretty slow. Note that since this particular model has (threaded) union connectors, if you get the same model as a replacement, you can swap it in without soldering.