Hikermor,
I have no intimate familiarity with the trekmate, but I did get to use plenty of MRE flameless heaters, both as a Naval Reservist and when camping and fishing in Georgia. The issue with flameless heaters is hydrogen gas given off during the exothermic reaction. They also will produce steam which some may take for smoke.
http://en.wikipedia.org/wiki/Flameless_ration_heater