Going into Stealth in a crowd in 5e

I recently ran an encounter on a “restaurant boat” where my players had to obtain info from a noble on board. They successfully bluffed/bribed/sneaked their way on board. Certain areas of the ship were off limits to guest, such as the crew’s quarter’s, the captain’s cabin, and the lower deck with rowers moving the ship along.

My players found a somewhat quiet spot on deck. One of them cast Pass Without A Trace and they all rolled for stealth. No one got less than 19. They then proceeded to “sneak” past guests into the upper areas (not forbidden per se) and then further into “Staff Only” areas.

My question is: how would I rule this? Obviously, they pose as guests, no weapons or armor on, so they would just blend into the crowd and then, in an opportune moment, sneak past a door/curtain/rope barrier. But they are still moving in plain sight of at least a dozen NPCs. They can probably see them, but they do not perceive them.

The same problem would arise for me if they wanted to escape someone following them in a dense marketplace or in a crowded tavern. How can you Hide/Roll for Stealth in a crowd of – admittedly uninterested in you – NPCs?