I just experienced a minor mystery with my Alt light.  I turned the
ignition on without paying attention but got distracted before
starting the engine.  When I looked to start, that Alt light was not
lit.  I went ahead and cranked, and the light lit briefly then went
out as the engine started.  Subsequent operation (five or six starts)
has been normal.

I shall reveal the trick as and when I discover it...

David