Odd electrical issue
Unpowered test light between the positive battery cable and the terminal .
The light works fine . I used it several times today
OK you are still not clear. You are saying you disconnected the positive cable and put the light in series, between the positive cable and the battery terminal? And "everything" in the car is supposed to be off?
I'm with Redfish here. I would suspect something aftermarket. Also, ALL cars have "some" draw, getting down into less and less milliamps. As your test lamp gets smaller, it is more sensitive. If this happens to be say, an LED test lamp, I could visualize a draw that would be "on the verge" of triggering a diode into conduction, thus flickering the lamp. I have a Fluke meter, EG if you check some LED individual diodes with that, it will flicker the LED sometimes.
I would measure the draw with your meter, and see what that indicates. I would guess based on your statements that the draw is very low, so be ready to get your meter down into the milliamp range. You might be "seeing" a diode in the alternator, or one of the radio suppression caps. Or as Redfish suggested. And I agree with you clock?