A standing pedal stroke is a pretty uneven application of power. That makes me inclined to think that the part that is noisy is more free to move about when you pedal more evenly. Chainrings are a often overlooked maintenance item. (For good reason, they rarely need anything done to them.) But a loose chainring bolt could cause some noise.
I'm also inclined to believe that the LBS did a good job checking your bottom bracket but that is pretty easy to do yourself. Speaking of easy to do, have you checked the hubs for play? Grasp at the top of the wheel and tug back and forth (gently, but firmly), feeling for any side to side play.
Also consider that when you're standing, you've moved up and away from the drivetrain. It's possible that it still makes the noise but you just aren't hearing it.