Several years ago I took a job working for a large telecommunications firm as the desktop support manager, and somehow I got assigned to the most loathsome of all duties in our shop: hardware inventory clerk. I was elected in true democratic fashion, being nominated and appointed while on vacation.
On her way out the door, the outgoing clerk told me that something strange was going on: In spite of her best efforts, inventory "irregularities" kept occurring. My boss, the IT manager, told me that the outgoing clerk's poor clerical skills were responsible. I was determined to do better, so I shifted from her weekly inventory to a daily one.
Sure enough, my inventories kept coming up short. One afternoon, right after a couple of new laptops had been delivered, I ran my inventory. The next morning I did another one, and discovered that I was two laptops shy; no machine deployments had taken place in between.
Only two people had swipe-card access to the hardware room: me and my boss. There was also a key in my desk drawer that opened the door, but using it set off an alarm at the security desk. I checked the automated access log, and sure enough, that key had been used at approximately 3 o'clock that morning. Apparently the alarm had sounded, but there was no entry written in the security log. Archived records showed similar occurrences during the past year and a half. I cross-checked the data and found a correlation between "incidents" and two of our guards -- who obviously didn't know about the automated access log.
Convinced that I had solved the case, I scheduled a meeting with the director of security. I presented my findings piece by piece, built my case slowly, and concluded with my theory that the two guards were thieves. I turned expectantly toward the security director ... and noticed a strange frown on his face. He declared that the evidence was circumstantial, gave me a hard look, and demanded to know how he could be sure I wasn't committing the crime myself!