First see if you have a short when the key is off.
Disconnect the ground wire from the battery and put a continuity tester between the battery and the ground wire. If it shows continuity, you then have to start disconnecting leads until the light goes out - that would be the connection/lead/device that is shorted and is draining the battery.
If this does not find a short, report back for further instructions/suggestions (which will be: do the above tests with the key on).
By the way, did you try starting the engine right after turning it off?
How much time elapsed before you measured the battery and tried to re-start?
Last edited by tclederman; 12/01/2013 7:11 PM. Reason: added step 2 instructions and questions
