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If you get the light when the car is running then it is likely a regulator/alternator issue. I've never tested to see if a really bad battery issue can cause this on a newer car. I know an older Porsche won't get the light with it's battery removed or completely shot. I've got firsthand knowledge of both of those.
A battery is a chemical/mechanical device. Fully charged it's basically lead plates and sulfuric acid. As the battery discharges the lead dissolves into the acid. This is normal and completely ok. If the battery is left discharged over a long period of time then the lead will collect at the bottom. The more lead on the bottom the less charge the battery can hold. At some point the lead on the bottom will short the plates out and then the battery is toast. The reaction is temperature sensitive so very hot or cold weather will change it's behavior.
My guess is the regulator/alternator is failing. The battery was probably discharging to keep the car running the first and second days. Once the battery got too low the load on the alternator caused the warning light to come on. Have the alternator and the battery checked at a parts place would be my recommendation.
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