There are two possibilites which explain your intermittant change indication.
Firstly, that light will come on prolonged when battery needs substantial charging but isn't getting it. (you knew that but why?). But if your battery were quite (95%) discharged and your rpm is low and/or the alternator is busy furnishing current to headlights or other systems and was unable to provide substantial charging current to the battery for several minutes. Recall that a you may have a 70 amp alternator but you can have car circuits demaning 70 or 80 amps . Part of this possibility, your battery may have had a lead denderite shorting two plates which later cleared under charging current. Also, my F-car, which I do not drive daily, the charge light come typically for a few minutes on first use, very normal.
Second possibility - there are 3 diodes in the alternator which convert ac to dc. Very often, one of these will fail and although the alternator retains some ability to charge, the current is substantially reduced and you find out later that you need a new alternator. That later can be a month. If the diode fails shorted, it can discharge the battery in hours. Also, you may have a condition whereby one of those diodes is intermittant.
The situation calls for replacement of both alternator and battery. Or if you aren't working for the sultan, continued observation to see which one needs attention. Intermittants only get worse, usually waiting for the most inopertune moment, such as being later for an early morning flight.
Incidently, electrical things work because they are filled with smoke. I know this because when I see smoke coming out of them they quit! Sorry, this book needs an index.
1993 MB190e 104k