User blog comment:Tiredman2/Mass Effect Plot Holes and Contradictions/@comment-24.76.84.236-20120824032415/@comment-24174486-20120824135354

Thermal clips were first used by the geth and implemented by organics after the events of ME1, so it's possible they started using thermal clips in the time between the Battle of the Citadel and the destruction of the Normandy.