Something that I have seen happen quite often that can kill your battery, especially if you don't drive it every day, is the glove box light not going off. It's one of those things you don't really check, and depending on how long it's parked, your battery could go dead as a doorknob. Also, if that battery has gone dead a couple times before, you may just have a battery that won't hold a charge (unless it's a deep cycle, like an Optima). Also, if it's dead now, it may be worth replacing anyway. A broken cell in the battery will do the same. It will show a good voltage until you load it down with headlights or an engine crank, and then you'll see the voltage just plummet. You won't do any damage to the ecm, ecu, bcm, etc... if you disconnect the negative terminal while it's running. Your alternator should be grounded to the chassis also, so the control module is still seeing a potential difference with the battery disconnected. I disconnect the negative terminal from time to time if I trip a check engine while working on a car, such as forgetting to plug in a sensor or something. It resets the brain. I think you should check the alternator with a meter. Just connect it to your battery and switch it to dc voltage. Then, start the car. If the voltage doesn't go up to around 13.5-14 volts after you start it and it's running, turn on the blower motor and your headlights, and see if it drops below 12 volts. If it does, your alternator may be bad. It should stay above 13 volts with a good alternator. Then, rev up the engine and keep an eye on the meter to make sure that it doesn't go above 14.4 volts. Unless you have something aftermarket installed in your car, excessive current draws that are not obvious are very uncommon. Wiring usually just doesn't go bad. Even bcm's and ecm's draw nothing for current unless they are doing something obvious like keeping the domelight on or parking lights, etc. Also, I don't remember how far back GM had the door trigger feature for the radio, but if you have that, where the radio stays on after you shut the car down until you open the door, make sure your radio (or domelight) doesn't stay on after you leave the car. Just throwing some thoughts out for ya. But, honestly, I think that it's either the glove box light, or just a bad alternator or battery.
