AzHonda had the proper test procedure with the test lamp. Hook it in series(like a fuse) on the negative side of battery. Any parasitic load on the battery will appear as a lit test lamp. Remove fuses or disconnect items, one at a time, until light goes out. This will locate the circuit.
The type of problem you describe can be caused by a backfeed, usually caused by some green fuzzy connection or a module or relay bad ground. Sometimes backfeeds can be the fault of something as simple as the two filaments of a bulb touching after the one breaks off the common lead inside the bulb. This would connect the park lamps to the brake lamps circuit, placing voltage on an unintended path.
FYI A fully charged battery will read between 12.7 and 13.2 depending on how much of the flash charge has been drained off.
A discharged (in need of charge) will test 12.5 on a voltmeter. Load testing can be done with either style of load tester that was shown in earlier post. Or you can rig up several headlamp bulbs as a load. What you want to see is a voltage that drops from the 12.7 to the 12.5 slowly, and will recover to somewhere over 12.5 after load is removed. Voltage from 13.2 to 12.7 will drop fairly quickly as this is the flash charge(surface charge).
Some modules, radio memory, ECU, FCM may have keep alive memory. Which can appear as a parasitic load, barely lighting the test lamp. If you leave the test lamp hooked up as described, and use a piece of wiring to temporarily jump across the test lamp leads. This should furnish enough current to turn off the module(draw). After removing wire but keeping test lamp connected, the lamp should stay out.
Low battery voltage, poor grounds, crusty green connections, added equipment can all cause a power drain situation. A process of elimination of curcuits is the only way you can find/fix these problems.