Oh man, I just had a look now, and now I can't find it. I think I read it under "special considerations" or it was on the actual combat chart pages somewhere.
It surprised me, because I didn't recall reading that rule in TTB, and could have sworn it was a Snapshot (or even BK4) only rule.
And yes, I do seem to recall pointing out to my trigger happy friend that panic fire with laser weapons was not allowed under Mercenary.
Just searched all four CT core editions for the word "Panic"
CT-77 not in book 1
CT-81 not in book 1
CT-83 (TTB) one hit, in the adventure.
CT-84 ST not in book 1, but again in the adventure in book 3.
AHL references morale only
Striker - morale only
Book 4 has the panic fire rule.