A B737 FLC RETURNED TO LAND WHEN THE ACFT WOULD NOT PRESSURIZE AND THEY HEARD A LOUD NOISE COMING FROM THE NOSEWHEEL AREA. MAINT PERSONNEL HAD BEEN WORKING ON THE ACFT AND HAD FAILED TO SECURE THE DOOR PRIOR TO THE FLC'S DEP. RETURN LAND.

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OUR ACFT ARRIVED AT GATE AND I PROCEEDED TO DO THE PREFLT INSPECTION. ALL WAS OK. AS I PROCEEDED TO FLT DECK BEGAN MY FO DUTIES OF PERFORMANCE DATA; CLRNC AND SETUP FLOWS. DURING THAT TIME MAINT CAME TO FLT DECK AND SAID THEY WERE CHKING THE LNDG GEAR WARNING HORN SYS. THEY DID NOT EXPLAIN ANY DETAILS OF THE TEST. AFTER EVERYTHING WAS COMPLETE WE PUSHED FROM GATE; TAXIED AND PROCEEDED TO TKOF. UPON LIFT-OFF WE HEARD A VERY LOUD NOISE COMING FROM NOSEWHEEL AREA. THE ACFT ALSO WOULD NOT PRESSURIZE; THUS; WE CONCLUDED THAT POSSIBLY THE E AND E COMPARTMENT DOOR MIGHT BE AJAR. WE HAD NO LIGHT INDICATION FROM THE MASTER CAUTION SYS SHOWING THE DOOR TO BE OPEN; BUT IT WAS THE ONLY THING WE COULD THINK OF. AFTER LNDG BACK AT OUR DEP ARPT; MAINT DISCOVERED THE E AND E COMPARTMENT DOOR OPEN. APPARENTLY THEY HAD OPENED IT WITHOUT ADVISING US AND FORGOT TO LATCH IT COMPLETELY. THIS HAPPENED AFTER MY PREFLT INSPECTION. MAINT SHOULD ADVISE FLCS ANY TIME THEY OPEN ANY PANEL ON THE ACFT AND SHOULD VERIFY TO US THAT IT HAS BEEN CLOSED; OR WE MAY HAVE TO DO ADDITIONAL PREFLT INSPECTIONS. SUPPLEMENTAL INFO FROM ACN 329945: I BELIEVE DUE TO DRAFT OF AIR AROUND NOSE DOOR WAS KEPT CLOSED SUFFICIENTLY TO NOT ILLUMINATE DOOR LIGHT. APPARENTLY DOOR CAN APPEAR SHUT WITH DOOR LIGHT OUT; BUT AT CERTAIN SPD UNSEAL DOOR FOR UNPRESSURIZED FLT.