Answer: Dumpster diving
Explanation: Dumpster diving is the method used in the computer field reviving the confidential and important information that can be used for attacking the operating systems. The information is retrieved from trash or dump as well as places like notes, sticky notes, codes etc.
The situation mentioned in the question describes about the dumpster diving because the note containing Id and password is thrown in trash. It is revived from there and can be misused by attacker for hacking or attacking the computer system.