I think it may also be for play but if you want to work on getting Marshall's full attention treats would be a great idea.
I bring treats on Rocky's walk EVERY day. It's not that he always gets a ton of treats - but that way I have them if he would somehow slip out of his harness or collar or both or whatever we are using that day.
When I see another dog in the distance I put Rocky in a sit stay. I position my body in front of him so that he can't see the other approaching dog. Rocky has known "watch me" since he was about 3 months and we still practice it in the house, etc. He is pretty good with distractions and will look in my eyes as long as I am pointing to my eyes, holding a treat, and saying "watch me".
Don't get me wrong - this isn't a flawless technique - but at least it would prevent the stare down that you are explaining that marshall does. He may still glance over at the other dog like Rocky does, but then the attention is usually right on me again and that is when I say YES and reward him.
Just an idea. Like you said - you don't think Marshall is exhibiting any aggressive signs but who knows how another dog would interpret it. Might not be a bad idea to start the look or watch me and physically interrupt his ability to visualize the other dog.
good luck - keep us posted!
Rocky, CGC 8/4/09 - Brindle - a docked and floppy boy :)
75% boxer, 25% st. bernard! "A boxer on stilts"
Dharma - our rescued senior, 8/18/03 joined our family 8/18/12