In our game, the DC is just the passive perception of the enemy, unless they are on active watch duty then they make a contested roll. That usually requires us fucking something up to put them on alert though.

I would also recommend Hell Let Loose. We recently had a game night with about 6 friends who all come from call of duty style, faster paced games. All of them mentioned how slow it felt and half of them were able to adjust to the new style and did pretty good. I would compare it to Battlefield but in a hardcore mode with less destructable environments.