What was found
(CAN) DURING CLIMB, FLIGHT CREW OBSERVED APU FIRE DETECT MESSAGE. AIR TURN BACK PERFORMED. A PRESSURE GAUGE/SWITCH IS INSTALLED ON THE APU FIRE EXTINGUISHER BOTTLE FOR BOTH A VISUAL PRESSURE INDICATION AND FOR A LOW BOTTLE PRESSURE INDICATION IN THE FLIGHT COMPARTMENT. THE PRESSURE GAUGE/SWITCH IS NOT AN LRU. FOR THE CHECK FIRE DET WARNING LIGHT ON THE CAUTION AND WARNING PANEL (CAWP), THE FIRE PROTECTION PANEL (FPP) CAN SHOW THE RELATED APU INDICATIONS. : APU FIRE APU EXTG APU FAULT APU BTL LOW. AS THE EXTINGUISHING FOR THE APU IS AUTOMATIC, IT IS POSSIBLE THAT THE PILOTS THINK THAT THERE WAS A FIRE BECAUSE THE APU BTL LOW CAUTION LIGHT CAME ON.