Marked keys? [New Player]

Hey fellas, new player at about 80hrs. What is the best way to get a marked key or am I going after the wrong thing?

I've seen on EFT related media that the marked keys usually give really good loot so I wanted to try and get one, I've seen a bit about the woods bunker so I've hit that about 20 times and haven't got one yet. Is there a better way to go after them? Or should I look for different loot until I level up a bit more?