H25B CREW WAS UNABLE TO PRESSURIZE THE ACFT BECAUSE AN AIR/GND SENSING SWITCH FAILED IN THE GND POS.

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WE DEPARTED TNCM WITH 4 PAX ENRTE TO KILM. CLBING THROUGH 11000 FT; THE PAX RPTED 'SMOKE' IN THE AREA OF THE BAGGAGE COMPARTMENT; WHICH IS LOCATED DIRECTLY AFT OF THE FLT DECK ON THE R SIDE. I TURNED AROUND TO CHK AND NOTICED THE 'SMOKE;' (WHICH TURNED OUT TO BE MIST/CONDENSATION FROM OUR LOSS OF PRESSURIZATION). WHEN I TURNED BACK AROUND TO CHK THE INDICATIONS IN THE COCKPIT; BOTH MAIN AIR VALVE LIGHTS (AIR BLEEDS) WERE ILLUMINATED; THE CABIN ALT HORN BEGAN SOUNDING; AND THE CABIN RATE OF CLB INDICATOR AND CABIN ALT WERE INCREASING RAPIDLY. UNAWARE OF THE EXACT SOURCE OF THE PROB; WE ELECTED TO MAKE AN IMMEDIATE TURN TO TNCM. I THEN NOTIFIED ATC THAT WE NEEDED TO RETURN TO TNCM. WE STARTED A DSNDING R TURN BACK TO TNCM. ATC ASKED US TO MAINTAIN 9000 FT AND MAKE A 360 DEG TURN. WE COMPLIED WITH THE TURN; BUT HAD ALREADY DSNDED THROUGH 9000 FT AND INFORMED ATC THAT WE HAD DONE SO. I THEN ASKED FOR PRIORITY DUE TO THE PRESSURIZATION PROB; BUT DID NOT DECLARE AN EMER. I INFORMED THE PAX THAT WE WERE RETURNING TO TNCM. UPON BEING CLRED TO LAND ON RWY 9 AT TNCM; WE MADE AN UNEVENTFUL LNDG. AFTER LNDG; THE TWR CTLR QUESTIONED THE NATURE OF OUR PROB. I REPLIED THAT WE HAD A LOSS OF PRESSURIZATION. HE MADE A COMMENT ABOUT MAINTAINING 9000 FT. WHEN I TOLD HIM I DIDN'T UNDERSTAND THE REASON/NATURE OF HIS QUESTION; HE MADE NO RESPONSE TO MY QUERY AND CLRED US TO TAXI TO THE RAMP. REFLECTING ON THE SIT; I UNDERSTAND HIS QUESTION. WHEN HE ASKED US TO MAINTAIN 9000 FT; WHICH ISN'T AN UNREASONABLE REQUEST WITH A PRESSURIZATION PROB; HE DIDN'T KNOW THAT ALONG WITH THE LOSS OF PRESSURIZATION; WE THOUGHT WE MIGHT HAVE HAD SMOKE/FIRE AND WANTED TO LAND ASAP. DRIVING MY DECISIONS WAS THE IDEA THAT IT IS BETTER TO BE CAUTIOUS AND LAND THE AIRPLANE SAFELY. THE THOUGHT OF FIRE INITIALLY CAME FROM THE PAX RPTING 'SMOKE.' A FACTOR IN THIS SIT WAS COMS. IT IS DIFFICULT TO COM SOMETHING YOU ARE UNSURE ABOUT. I WASN'T CONVINCED THAT WE HAD A SMOKE/FIRE SIT; BUT WANTED TO LAND QUICKLY; BECAUSE OF THE POSSIBILITY. PROB WAS CAUSED BY A FAILED SQUAT SWITCH.