All types of things can cause this. The very first 02' I did some electrical work on had a broken field sense wire to the alternator in clear view near the drivers side fender. That wouldn't quite drain the battery when sitting but would alter the output too low due to the inability to sense the current required to keep the battery charged.
Another potential issue; an alternator rectifier diode can fail and slow drain the battery with vehicle on or off. I had this problem on a vehicle of my own. With a meter on AC voltage, probe a ground and the alternator + with engine running (at operating temp). I had roughly between 15v - 30v AC.
The "alternat"-or (alternating current) does not initially create a direct current due to the design of the windings/rotor/magnetic field. Imagine merging lanes of opposing traffic to a one way street, this is the job of the rectifier diodes. When failure occurs in one or more diode, alternating current begins fighting back. I drove with one failed diode for awhile but others began to fail and the battery would drain over night.
Other possibilities: Poor connection between either battery cable. Poor ground from engine to battery and/or chassis. This means too much resistance. Using a meter on ohms (<-20), probe engine to battery negative, should read .06 ohms or less. Anything over .08 becomes questionable. Pos battery cable end to end should also read under .06. I all too commonly find poor connections.