Bob,
I’m not positive, but I’m pretty sure that putting a marker doesn’t do anything. If there is no more media I think the playhead just stops. I don’t believe that it continues to chase after markers.
I usually just drag my audio out 1 more frame than the video. That gives me my frame of black and since the audio is fading out too, who’s gonna notice one extra frame? (This is non-broadcast work, obviously.) Putting a slug in works too, but my way is a little bit faster so that’s what I do.