You're referring to quantum effects? Don't worry about whether you're not watching, the universe is watching. If one photon is emitted from the thing in a quantum state and hits anything, that's the observation

A lot of the confusion around quantum mechanics comes from misleading cartoons about the double-slit experiment which don't occur in reality. They usually depict it as if the particle produces a wave-like interference pattern when you're not looking, and two separate blobs like you'd expect from particles when you look. But, again, you have never seen that, I have never seen that, no physicist has ever seen that. It only exists in cartoons.

In fact, it cannot occur because it would violate the uncertainty principle. The reason you get a spread out pattern at all is because the narrow slits constrain the particle's position so its momentum spreads out, making its trajectory less predictable. There is simply no way you can possible have the particles both pass through narrow slits and form two neat blobs with predictable trajectories, because then you would know both their position and momentum simultaneously.

What actually happens if you run the calculation is that, in the case where you measure the which-way information of the particle, the particle still forms a wave-like pattern on the screen, but it is more akin to a wave-like single-slit diffraction pattern than an interference pattern. That is to say, it still gives you a wave-like pattern.

It is just not true that particles have two sets of behavior, "particle" and "wave" depending upon whether or not you're looking at them. They have one set of equations that describes their stochastic motion which is always wave-like. All that measuring does is entangle your measurement device with the particle, and it is trivial to show that such entanglement prevents the particle from interfering with itself when considered in isolation from what it is entangled with.

That is all decoherence is. If you replace the measuring device with a single second particle and have it interact such that it becomes entangled with that particle, it will also make the interference pattern disappear. Entanglement spreads the interference effects across multiple systems, and if you then consider only subsystems of that entangled system in isolation then you would not observer interference effects.