Remote locking unlocks and immediately locks again?

[teapot6/12]

Help! My new to me 2001 Foz 2.0 Sport has a central locking issue. When I bought the car both key fobs had no batteries in and there was also a separate one button "Subaru" badged fob which I now believe to be the Sigma alarm as there is a key pad in the glove box. I fitted new batteries to both key fobs and the car now unlocks when I push the unlock button on either of the fobs but immediately locks the car again? The alarm fob seems to do absolutely nothing and as I don't have the pin code to re-set it I can live without the alarm. I would point out however that when locked the red LED key sign flashes (immobiliser I think) and a separate red LED on the dash. I am assuming that the previous owner may have had issues with the keyless entry and has somehow disabled the Sigma alarm and removed the batteries from both fobs as it was doing what it now does to me! Help greatly appreciated! :-)