If you are able to check the state of charge on the battery after driving car for a while, and it'* indicating full, or near full charge state, then you may rule out the battery, alternator,and charging system as being faulted. The specific gravity of the electrolyte tells the whole story. Specific gravity in a lead acid battery (6 cells) should be 1.280 at 80 degrees faranheit. If the ambient temp is above or below the 80 degrees, there is a correction formula to solve for that to give you a corrected specific gravity. The problem with todays auto batteries we all know is they're sealed. Maintenance free, yeah right.
Some auto service places have "battery checkers" that actually check the battery under load so that they can determine "drop & recovery" so to speak.
It sounds like you had some intermittent cycling load on the electrical system for a short duration, then goes away. That is what you need to isolate. The alternator must put back into the battery all ampere/hours that were taken out by the cars electrical system (ie. starting engine, radio, A/C, lighting, other acessories) then float the battery. In other words when the battery recovers form the initial starting of the engine (alternator charging) they're running in parallel, if you notice lights dimming momentarily, it means that something is heavily loading the system. Hope this helps a little, didn't mean to get carried away, just an old submarine electrician here.
2003 SSEi, Light Bronzemist Metallic - All Stock For Now
IBEW Local 261 Groton, CT
My other vehicle is a nuclear submarine.