I hadn't noticed any leaks as my cruiser stern deck bilge is often covered in water.
Eventually after six months of hardly any rain and I still had wet bilge, I started to look a bit harder.
I also had a split in the insulation and the spotted water leaking from it.
Having removed the calorifier and the insulation discovered a bulge on the back side next to the hull, I initially suspected the pressure relieve valve and claimed from my insurance for a new calorifier due to a faulty valve.
It was rejected but the rep said it would only be covered for frost damage. It then struck me that I had been frozen in for weeks when the temperature dropped to -18 in December 2010.
I then claimed it was through frost damage which it probably was as it had bulged next to hull side where it would have been coldest. They eventually paid for its replacement.
So it could be that the OP's calorifier was damaged last winter and it has taken this long to show its self as did mine.
Alex