It is very common to have TA when flying over a place where small aircraft are, without C mode. Of course your TCAS does not know the altitude of the aircraft in question so it passes out an advisory. Of course it needs the other altitude to figure out any RA. So, not so strange at all, but I would think that APP control should have warned you about the other traffic, since you were IMC.
The intruder likely didn't have mode "C" engaged. We used to get it all the time up north where it wasn't mandatory to even have a transponder let alone mode C.
How on earth did you get TA if the other one didnīt have transponder?
