Thanks everyone for your answers.
So far I've decided to do the following:
I ask them to roll Etiquette or Stealth skill rank + Sin level vs Device Rating x 2. But only if their Sin rating is strictly (<) below the "device rating of the area".
I consider Downtown to be a 3-4 device rating area, Bellevue being 6 and Barrens being 0
If it's a tie or better they're good
If they lose by 1, recheck
If they lose by more than 1: alert and the Sin is burned.
If a character is Unlucky or Lucky it can have some repercussions on that roll.