A police officer or other witness can testify to the fact that he/she witnessed you eating, for example, what appeared to be a bag of powder.
Should you be tested for drugs (e.g. in a DUI situation) and the result turns out positive, the appropriate charges would be brought against you at that time.
I don't think tampering with evidence charges would be completely out of the question, either.
I'm sure there are situations like those you described in which the charges didn't stick, but in the real world I wouldn't count on it.
Legal issues and law enforcement are embellished routinely in the name of creative license; anyone who's ever watched Cops or Law & Order will certainly agree.
Further, in some situations the evidence can be recovered. As an example: throwing a bag of weed into the ocean is no guarantee that it won't float to the surface 10 feet down and into the clutches of a waiting officer.
It is usually more difficult to prove a case with circumstantial evidence than physical evidence. However, it can be, and has been, done many times.