I found this problem while playing Soldat 1.2; guys often go "invisible", meaning that it is not possible to spot enemies especially when they are close. Often they just appear out of abolutely nowhere and kills you.

That is because you are playing on a server with realistic mode enabled. That invisibility is 'Fog of War'. You can only see short distances. Yeah.

realistic mode.

if your playing real mode, look all around you, you cant see behind yourself or behind most polys.

it might also be a predator. theyre not invisible, but pretty damn hard to see