Oh, I forgot to mention Effort!!!
If a central element of a room is a contest of skills/stats…effort is key!
Roll a timer dice for rounds that the mobs will act on suspicion. Then contest effort vs effort (thief has easy rolls and tool/weapon effort for stealth. A mage with an illusion spell might have magic effort. An NPC with a magical eye might have magic effort.
A fighter in full plate bristling with weapons and a shield might have a hard roll and halved basic effort!
This highlights weaknesses characters may have, increases the tension in a non-combat situation, plants the seeds of consequence. It also unsettles the players by adding an arbitrary mechanic…that has logical merit. It raises the stakes in their minds. Hero coins get used for ultimate effort. This might be the boss fight! Or the fight before the boss fight…or an encounter against vigilante, but otherwise unremarkable opponents The players don’t know, other than the amount of time into the session.
How important is it that we stealth past these guys? The GM complicated it so it’s probably important! It’s the second scene, how important can it be?
Don’t interrupt their discussion but rush them. This is the psy-op portion of GMing. Allow the players to create doubt while you seem to just be keeping them on task!
“Huh, I wish I thought of that” with a sarcastic grin and tone, is one of my greatest GM tricks.
Did I already account for that possibility? Is that it exactly? Did he not even think of that, and we will now encounter it later?
The only time my players think they know when I think it was a good idea, is when I give them a Hero coin for the concept! But I have used that to continue a branch of thinking on the players side, just cause it fit what was coming next perfectly!