Hi,
This could be one typical reason which I experienced before - Condensation at the shutter.
And the other is yes, you left the camera setup open without covering and though it is 5 degrees and most cameras should not have a problem. But when the breeze comes and with the metallic construction of the body and lens, tendency that the accumulated temperature within the body could have dropped to well below the ambient temperature.
Did you try again now that you are back in Singapore?