This can be confusing, so let's reason it out.
Everything here is from the references shown below, with a few comments in an alternate color.
Engine Fire Loop Alert
Just the CAS, no light in the fire handle or fuel control switch.
Since engine fire warning (FIRE handle lights and FUEL CONTROL switch Lights) are dependent upon sensing fire / overheat in both loops simultaneously, the possibility exists that a loop could fail undetected or un-annunciated, or that a system fault could develop which causes a signal output similar to a fire warning (i.e., lighted LOOP A or B red fire test light), but not both. In these cases, fire could develop without triggering fire warnings, or false fire test light could result, again without fire warning. The pilot would be alerted to a single activated fire loop by a LOOP switch segment illuminated red on the FIRE TEST switch on the cockpit overhead panel, coupled with an Engine Fire Loop Alert warning message displayed on CAS, and the associated tones. Should this occur, re-test the system. If the system does not check properly, the loop has failed and the fire indication is genuine; execute the appropriate Engine Fire checklist. If the system does check properly, the LOOP warning is false; disable the faulty loop.
[G450 AFM, § 4-11-10]
If normal fire test indications ARE displayed:
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
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