With APU Fire handle pulled (P8 panel) and the Master Bright/Dim and Light test switch set to Dim, the fire warning horn sounds in the cockpit. The Engine fire handles and the remote APU fire handle remains stowed. With the Master Bright/Dim and Light test switch set to Bright, the fire annunciation and horn stopped. This complaint is attributed by a “sneak circuit” inside the APU Control Module (M280). The aircraft manufacturer has released a component service bulletin (CSB) 65-52801-49-08 which include a modification from MOD “A” status to MOD “AB” of APU Control Module (M280) p/n: 65-52801-67. Thanks to Jan Hogeveen.