Hi, we had this happen today. The TCAS displays the appropriate symbol without any altitude information. It treats the intruder aircraft as being at all altitudes so it'll give a "TRAFFIC" warning when the intruder is calculated to be a threat regardless of the actual height of the intruder (obviously the TCAS unit doesn't know how high the Mode A aircraft is and so acts on the worst case scenario.)
We had a TRAFFIC alert today from a Mode A helicopter 4,500" below us.