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The best way to check for this type of a problem is to put an amp meter inline at the battery and see if there is an excess draw, the PCM has a very small drain, 0.05amps. If you find more then make sure you don't have a light, etc on, then start pulling one fuse at a time, remember you have two fuse boxes, until you find the problem circuit.
If you don't have a multimeter to measure for excessive parasitic drain, which could be any of the things Ken outlined but could also include shorted battery cell/s, shorted or leaky alternator diodes, which are more dfficult to test for, why not consder running this ride by your favorite autoparts store, for a no cost electrical system checkup, via their portable electrical system tester.
It can properly load test the battery & alternator in the vehicle, to detect weak battery cells or alternator diodes, as well as check the electrical system for excessive parasitic drain.