If the pressure switch was turning off before you winterized, then it may be that there air in the system "in front" (i.e., between the pump and the FW tank[s]). I have notice that issue occasionally when I've winterized. If that's not the case, then it may, or may not, be related to a leak. It's possible that you have an internal issue with the your fresh water pump or just it's pressure sensor. WHAT KIND OF FRESH WATER PUMP DO YOU HAVE? Do you have a pressure accumulator tank?

As Westcoaster indicates, you can try an follow the trail back to the "leak". I've found that sheets of white paper towels placed in the path of travel will usually reveal an stain from a leak. Put the "traps" down after you've drained the bilge and dried it out. Then check them.

Is it possible that you have a leak from the head? Did you winterize the head's lines and possibly have a spill from there?