Hide behind objects with a flamethrower, like the left steps on highschool survivor, the left dumpster on checkpoint, ect. i use 3 so they cant mark me. also free shiv and easy shiv when someone walks by me, also covert combined with flamethrower is perfect for guarding the right tunnel on bus depot.
I have Covert on every build. If not for stealth, then for the free shiv, which is always handy to have.
Most of my builds have Covert 3 so that I can't be marked by anyone, so that enemy team has absolutely no idea where I am. This is a huge lifesaver when playing against teams that don't use mics. If they do, then I can at least get a jump on them, but getting rid of them after I kill someone is much harder, yet still possible.
I use this combined with marathon runner, to mislead the camping enemys, for example I first run straight in the direction of them let them think I am in front of them. Then i turn immediately around and flank them from behind. I met people that accused me of cheating because they thought they spotted me and wondered how i could get behind them so fast.
Covert 3 is not so useful because you cant hear them scream when someone marked you, I have a covert2 strategist 3 combination in on one of my loadouts. This is the best loadout to flank a team and crouch around.
Covert is a good skill i have Covert2 in all 4 of my loadouts, however a crouching person is more vulnerable to get attacked with shivs or guns from behind than a rusher, so it is necessary to find the right mix and not crouch the whole game around!.
I have 1 stealth Loadout which I use very very rarely. I like to use Crafter 3 and other perks more. I find covert a bit overrated, espeically Covert 3, because of the things already mentioned:
Others KNOW that you are using Covert because the can't mark you.
You will not know if you get marked because you don't hear the yelling.
One good point is the free shiv though, that is worth it, especially for interrogation mode.
If you don't camp but keep moving as a team and be careful with corners and typical "hiding spots", and especially check the surroundings before opening a box and crafting and have someone watching the back and the side, Covert users will not be able to do that much because they have to move so slowly and won't keep up. You can flank without Covert as well, if the enemy is occupied in a gun-fight. You can even sprint to flank because they don't recognize it as they don't pay attention to the mini-map when fighting. In the "worst" case all of them see your red dot and panik turning to your side which gives some opportunities to the rest of your team to move in.
Also, what I experienced, if someone with covert is flanking, he is often too late. So it is basically 4v3 because one is crouching around somewhere trying to get behind us. Until he reaches us, it is all us against him..
I usually get more kills when playing "non-stealthy" than when playing "stealthy". I mean yeah, it might happen that you get 4 guys crafting on a box with a molotovs, but that only happens against noobs and very rarely.
I really love shiving Covert3-Users though. Very satisfying