Have I had the ‘mystery key’ issue? Yes
Have I had to physically break and drill out locks on NAS devices, servers and cabinets as a result: Yes
Have I found missing / long ‘decommissioned’ equipment still in place and the new replacements missing: Yes
Do I regularly find work that was never completed but was documented as being so: Yes
Do we have regular disasters caused by the tweaking of systems / changing of credentials to something not documented: Yes - see my post in this thread .
In short, we had a (thankfully, now ex colleague) who was a total nightmare. This guy was an information hoarder and deliberately went out of his way to keep key information and procedures to himself. When questioned and asked for key data, he saw it as a ‘threat’ / challenge to his obvious superiority over the rest of us. If asked ‘Bob whats the login details for ABC’ or ‘How does XYZ work?’ he would respond saying ‘No its Ok, I’ll do it’, in an effort to make himself indispensable and keep everyone else out of the loop, so we essentially couldn’t work on / fix certain systems.
When other people were equally / better skilled than him at something, he would take it as a personal insult and huff about it. Rather than co-operating and working as part of a team, he would isolate himself and go out installing / fixing / doing things by himself for days at a time. Even now a few years later we are still finding things out in the field that he claimed for days and weeks of overtime to complete, but we find the work was never done. He saw the day to day operations of the department and things like dealing with users as something that was beneath him and not his concern, and would pick and choose the fun little projects he wanted to be involved in and would disregard the mundane / hard stuff and everything else. He would make ad-hoc changes to critical systems and would then deny it when things screwed up and crashed, and would even go so far as to do the whole hero ‘Look at me, I fixed it!’ routine to management when he undid whatever change he shouldn’t have made in the first place. He treated the network like his own personal little lab to play around with and learn things. He constantly arrange overtime for ‘maintenance’ and would do no quantifiable work, he could be found sitting reading a book or watching Netflix. If other members of the department offered to stay and help he would either plain ignore them and not say what he was working on and needed help with, or fob them off by saying stuff like ‘Its really a one person job’. Then he would complain to management that he got no help and support from colleagues.
He was a very clever manipulative individual and for numerous reasons which I wont go into here, management seemed to have a total blind spot to what he was doing. One by one, every other member of the department voiced their concerns but it fell on deaf ears for a long time.
Long story short, he is no longer there, and although we are drowning in work, the atmosphere is just 100 times better than it was.
When he left he formatted his machine and I had to do a forensic recovery on it, on there we found tons of his own personal documentation where he had changed the passwords for numerous things to something different than what was in our documentation. From this we found some, but not all, the missing passwords and setups.
But, it’s just the gift that keeps on giving, not a month goes by when we don’t find something he interfered with, screwed up, or re-did in his own image, because ya know, his way was always ‘better’.
We’re slowly but surely finding and fixing everything he painstakingly fucked up.
From previous admins with love indeed 
