During preflight it is noted that placards are missing on the avionics nose wheel access door; indicating position of the door latch locks. Door was found unlocked with no appropriate warnings in the cockpit.

Narrative

Exterior Preflight: Standing by the nose wheel; looking forward and up into the wheel well and viewing the avionics nose wheel access door-- I observed it was painted completely white. Unlike other aircraft in the fleet; this aircraft did not have placarded decals to verify door handle latch positions; (CLSD or OPEN). There was no interior cockpit annunciator illuminated suggesting that the access door was unlocked. Note: This unlockable door yields alternative cockpit access from the ground; when electrical power is not available to operate the normal aircraft doors and/or maintenance is not available to mechanically operate normal aircraft doors. In any event; I have always confirmed the handle latch position in relationship to the placards of CLSD or OPEN. Because this aircraft did not have placards; I logged the discrepancy in the maintenance log assuming they were required. After approximately 45 minutes; Maintenance personnel returned and explained they had searched every reference source possible and no one could find a clear placard requirement for that door. He was very professional and polite. I asked only that someone please crawl down the cockpit interior floor access well and verify that the door was indeed latched properly for flight. The mechanic agreed and sent an assistant down the cockpit access well into the avionics compartment. The door was found to be NOT LATCHED properly. The mechanic corrected the problem by securely latching the door. The mechanic documented all in the logbook. Note: Another aircraft on the ramp was checked for the door placards but also had none. After completing this flight; I checked another aircraft on the ramp and found it had the placards CLSD & OPEN on the door. I think earlier modified (Passenger to Cargo) aircraft could have had a different contractor than did the later aircraft series. Note: Previously I had experienced a problem after engine start. A mechanic came aboard the aircraft by accessing the cockpit via this nose wheel well door. In addition; with the engines running I absolutely remember that a warning light was annunciated when the door was breeched. The systems manual list two alerts: 1) AVNCS EXT ACC DR (MISC)-Avionics external access door not closed/locked; 2) AVNCS NOSE WHL DR (MISC)-Avionics nose wheel door not locked/closed.