B737-300 CREW HAD THE CABIN ALT WARNING HORN GO OFF. THE ACFT WAS NOT PRESSURIZING. BOTH PNEUMATIC SWITCHES WERE FOUND IN THE OFF POS.

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UNEVENTFUL DEP FROM ISLIP; VARIOUS VECTORS FROM ZBW IN THE CLB FOR SEPARATION. PASSING APPROX 14000 FT THE CABIN ALT WARNING HORN SOUNDED. THE ACFT WAS LEVELED AND A DSCNT TO 10000 FT WAS REQUESTED AND APPROVED. LEVEL AT 10000 FT THE QRH FOR CABIN PRESSURIZATION WAS COMPLETED. THE DECISION WAS MADE TO DIVERT TO BWI FOR MAINT. WE CHANGED ACFT AND CONTINUED ON TO FLL AND RETURNED TO BWI UNEVENTFULLY. I WAS CONTACTED OCT/SAT/04 AND NOTIFIED THAT SUBSEQUENTLY THE ACFT FLT RECORDER DATA HAD BEEN REVIEWED AND IT APPEARS THAT WE MAY HAVE DEPARTED ISLIP UNPRESSURIZED. SUPPLEMENTAL INFO FROM ACN 635043: LOOKING UP; I NOTICED THE BLEED SWITCHES IN THE OFF POS; BUT I THOUGHT THIS WAS NORMAL AS THE CHKLIST CALLS FOR TURNING THEM ON ONE AT A TIME. IN HINDSIGHT; I BELIEVE THEY WERE OFF THE ENTIRE TIME INDICATING AN UNPRESSURIZED TKOF FROM ISP. AFTER RECENTLY DISCUSSING THE MATTER WITH MY FO; HE RELAYED TO ME THAT HE THOUGHT I HAD POSITIONED THE ENG BLEEDS TO THE OFF POS WHEN WE WERE AT ALT. IT'S CLR NOW; THAT WE TOOK OFF UNPRESSURIZED. THOUGH WE ESTABLISHED CREW COMS AFTER DONNING THE MASKS AND CALLING FOR THE CHKLIST; I NEEDED TO ASK HIM MY QUESTION CONCERNING THE BLEEDS IN THE OFF POS. HE NEEDED TO ASK IF I HAD PLACED THEM THERE. I TRULY BELIEVED SOMETHING WAS AMISS WITH THE PNEUMATIC SYS. NOW I KNOW THE ENG BLEEDS WERE OFF FOR TKOF.