Imagine harder. Can you imagine a cop being worse at their job than everyone you currently work with? If people could not take things from evidence, evidence would not go missing. It happens less in larger departments that have more secure protocols for evidence handling, but in those cases, the stuff mainly gets stolen by evidence tech employees that know better how to cheat the system. It disappears just as surely as merchandise is stolen from Wal-Mart by its own employees.

Even if it was not evidence from another case, I don't think that new glass drug pipes are hard to obtain. After all, cocaine addicts that have smoked up everything else in their life can still get them. Cops have closer physical proximity to that lifestyle, and could pick up an abandoned pipe from one place, and later drop it where it would be more useful.

This type of misbehavior has been documented on bodycams, when the cop thought the recording device was on standby. Drop contraband, turn on bodycam, "find" contraband, arrest suspect for possession of contraband. Cops that cut corners to take down drug dealers--or other people whom they just know are guilty--think they are doing the right thing.