I think you'll find that the transponders found on ships and other surface vessels are not Mode C equipment, and only return a Mode A signal (why would a ship have an altimeter?)
Consequently, return from these will be Other, Proximate or TA only, and will not have a level assigned to them on the TCAS display.
I would not expect a WX radar to result in a TCAS return, for two reasons:
1) Different frequency bands (Transponders on 1030MHz, Weather radar > 4 GHz)
2) Transponders work on the PRF of the response in order to segregate signals. Weather radar is not going to generate PRF of the same nature that a Transponder will.
That's the limit of my radio navigation wisdom - without further info on your unit or the specific circumstances, my only conclusions can be that it was an aircraft transponder being operated Mode C on the ground, or your unit has a fault.
Icarus