[bubblethink]

After they get it when they are authorized to get it. You cannot point at random things and call them evidence. For instance, I can take a dump and flush it, despite law enforcement's assertion that my stool is evidence in some imaginary crime.

[kube-system]

It doesn’t have to be something the police have already collected. It could be something that they have yet to collect.

The textbook tampering with evidence example is: drug dealer tries to flush the drugs when the police knock on the door.

[bubblethink]

Yes, when they are authorized to make a drug bust. And that's assuming they catch him before he flushes, i.e., get the drugs. Otherwise, what are they going to say? He is a drug dealer because we heard the flush? Or let's say they are doing a missing person's search and hear the flush. What now? We came looking for a missing person, but heard the flush, so there were drugs whose evidence he destroyed?

[kube-system]

Of course, tampering with evidence is — itself - a crime. And you need evidence to prove that crime.

But yeah, people tamper with evidence because they think they can get away with it. That doesn’t mean they always succeed in cleanly doing it without creating evidence of the tampering itself.