It's more similar to finding a key hidden under the mat at someone's house. You can then contact the owner and inform them of the security issue but what you should not do is use the key to open the door and go in and see if there is really harm in you being able to enter. Because you might accidentally achieve the exact thing that a criminal wants such as finding a note with a password on it. You can then claim you didn't want to find it but the fact is that 1) you broke the law by entering and 2) you caused a malicious event (namely obtaining a password).
You can then pinky swear that you didn't already use it for any further malicious actions but that will be difficult to verify.
If I ever lose my key, I don't want people to enter my house to prove that they can. Inform me of how you obtained the key and I'll change the locks and make sure I don't lose my key again. If you do enter my house, expect me to press charges.