X-ray lasers would be neither quiet nor subtle; because air is opaque to X-rays, the X-ray laser must tunnel through atmosphere, which will create a plasma streak from weapon to target, similar to a straight lightning bolt.
Near IR lasers, operating at energy densities low enough to prevent atmospheric ionization, wouldn't normally be visible to the naked eye, though near IR sensors wouldn't have much trouble picking up the beam path based on scatter and reflection, and in dusty conditions incinerated dust might make the beam visible. A near IR sniper rifle, however, would tend to have short range and a rather large focal array.