I'll bet you didn't have any film in the camera when you had your "play around".

Correct me if I'm wrong, but I believe that the OM40 has two metering systems - one to give an indication of exposure in the viewfinder, and another that actually controls the exposure by metering the light bouncing back from the film and shutter curtains.

If you try some test exposures in lower light without film in the camera, the black pressure plate reflects much less light than film, and as a result the exposure is lengthened.