B737-300 FLT CREW DISCOVERS ACFT CLEANERS HAVE USED EMERGENCY EXIT LIGHTS FOR ILLUMINATION WHILE CLEANING ACFT.

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WE BEGAN OUR PREFLT DUTIES AND COMPLETED THE 'BEFORE START' CHKLIST. AS THE CABIN DOOR WAS SHUT; THE FLT ATTENDANT CAME UP AND ASKED IF SHE COULD FLIP THE EMER LIGHT SWITCH DOWN. I THOUGHT SHE MEANT THE GUARD OVER THE SWITCH AND SAID I THOUGHT THAT IT WOULD BE OK BUT TO ASK THE CAPT TO MAKE SURE. THE CAPT WENT BACK TO CHK OUT THE ISSUE. I WAS TRYING TO GET THE PERFORMANCE DATA READY AND WAS TALKING TO THE GND CREW EXPLAINING THAT WE WOULD NEED A FEW MORE MINS. THE CAPT CAME BACK UP AND SAID HE HAD TESTED THE LIGHTING BY TURNING THEM ON AND BACK TO NORMAL A COUPLE OF TIMES AND EVERYTHING WAS WORKING PROPERLY. OUR UNDERSTANDING OF THE SYS IS THAT WHEN THE LIGHTS ARE TURNED ON THAT THEY ARE USING THEIR OWN BATTERIES REGARDLESS OF PWR ON THE ACFT SO WHEN THE LIGHTS CAME ON THAT THEY WERE CHARGED. WE DEPARTED FOR OUR DEST. AFTER FURTHER DISCUSSION; WE DETERMINED THAT THE CLEANERS HAD LEFT A NOTE SAYING THAT THEY HAD USED THE EMER LIGHTS TO CLEAN THE ACFT BECAUSE THEY HAD NO PWR. THEREFORE; THE EMER LIGHTS MIGHT NOT HAVE BEEN FULLY CHARGED WHICH WE HAD NO WAY OF KNOWING. THE CAPT ALSO STATED THAT THE SWITCH POSSIBLY COULD HAVE BEEN CAUGHT IN A MIDDLE POS. I THINK THAT THE BEST THING TO DO IS IF ANY SWITCH IS OUT OF THE NORMAL POS FOR EMER EQUIP IS TO CALL MAINT CTL JUST TO BE SURE OF THE SITUATION. ALSO; THE CLEANING CREWS NEED TO KNOW THAT IT IS NOT AUTH TO USE EMER LIGHTS FOR ANY REASON! FINALLY; WE SHOULD HAVE STOPPED AND ASSESSED THE SITUATION FURTHER EVEN IF THAT MEANT DELAYING THE FLT; TO MAKE SURE THAT EVERYBODY WAS ON THE SAME PAGE. I FEEL IN TRYING TO GET AN ON-TIME DEP AND FINISHING MY PREFLT DUTIES; I MAY NOT HAVE FULLY HEARD THE FLT ATTENDANT. NEXT TIME; I WOULD STOP AND GIVE THEM MY FULL ATTN IN A SITUATION LIKE THIS.