A piece of Green-K that has been sitting anywhere -- but not encased in lead -- would be continually emitting waves that would be visible to Superman if indeed he is able to see its radiation.
However, if someone whips out a piece of K while he is nearby, the light from the green K reaches Supie's eyes at the same speed as the radiation from the green K reaches him. Thus he can be surprised. HOWEVER, if he can fly faster than light without flying blind (see below), then he would never be surprised by green K and could always escape it.
Flying Blind: If Supie flies at, say, ten times the speed of light, the light (from any object) that reaches his eyes is moving at the speed of light. Therefore, if the object is moving rapidly he would be unable to see it before passing it or slamming into it. At any rate he would not be able to pinpoint its location until he slows down. In particular, if there is any range at all to his ability to see then there is the potential (depending on his speed) that he does not see a "dark" object at all until he hits it.
Solution: This is resolved by allowing Superman to see things by observing not through light but through tachyonic particles moving far faster than light. There could be many of these traveling at different rates, so that even if Supie races across the galaxy at 3,628,800 times the speed of light, he might be observing the objects around him through particles that move at 4,000,000 times the speed of light -- so that he sees the objects where they are. However, if Superman is able to see via tachyonic particles, these would be of such high energy that they would likely be unimpaired by the earth's atmosphere -- or even the sphere of the earth itself. Thus, he would never be surprised by Kryptonite anywhere -- rendering it effectively useless against him unless it emits tachyons.
If truly he is unable to "turn off" his vision powers, then most objects not containing lead would appear to him as x-rays do to us. He would see "ghostly objects" everywhere, as though he saw everything threw an airport scanner. Being able to "focus" would not change this, unless we mean that he could "de-focus" on everything else. This would be effectively the same as shutting off his vision powers. In all the early comics, X-Ray vision was clearly a power that he had to activate.