You may have an intermittant short.

Do you remember using any electrical device at the time?

Try to isolate parts of the loom by pulling non-essential fuses and drive around for a bit. If it doesn't happen, start putting them back in again one-by-one. If it happens, check that the red light corresponds with a low voltage reading across the battery.

You may find that it was a one-off, caused by say, going through a puddle and soaking the belt.

If it doesn't happen again, I wouldn't loose sleep over it, life is full of unexplained events............

__________________
Paul Gibbons
'93 320CE
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike