Star Trek VI: The Undiscovered Country by BarkerC

Question 10

So the cloaked bird-of-prey that fired upon the Kronos One is lying in wait for the Enterprise. While spouting Shakespearean quotations at Kirk during battle, the bridge crew realizes they can find the bird-of-prey by modifying a torpedo to search for what, expelled by any space vehicle under impulse power.

ionized gas or plasma exhaust