Well, you'd likely be wrong! One of two things, both related to the expansion tank: it either has a blown bladder, or lost most or all of its air precharge. My guess is that if you knock on it, you'll find it sounds and feels like it is very heavy (i.e., full of water). It should be mostly full of air. They're simple to replace unless they didn't install a shutoff valve, then it gets a little messier.
They come precharged with around 14-15# from the factory, but it is always good to check them before installation. Fill it carefully with a bike pump or small compressor. Carefully because they don't hold all that much air.
A quick check is to remove the cap on the schrader vavle and see if water comes out. That is supposed to be the dry side of the air bladder. If no water comes out (it probably won't until you press the valve down inside), then you'd need to check the precharge. Can't do that while there is pressure on the wet side, so you need to shut things down, including the water makeup valve, then drain a little bit out. Check and adjust the pressure.
My guess, though, is that the bladder is shot. They usually last longer than that, but it happens.