disconnect the ground from the battery (after a charge), then connect a 12 volt test light between the ground pole of the battery and the ground wire. if the light comes on you have a drain. Make sure nothing electrical is on(open door trunk lid. etc) .Open fuse box, remove fuses one at a time until the light goes out. The fuse that makes the light go out is the circuit that is draining your battery. Read what that circuit controls and start your search.
Faulty alternator or regulator ??
Much to my surprise my battery discharged after the regulator failed (worn out) .. and the warning light had not come on ..
Had you noticed the horn becoming weak, or the indicators slow prior to the event ??
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