I don't know much about real-world forensics, except for what I see on television, but one thing I do know is that when you're faced with a troubleshooting mystery, you have a couple of choices: shrug your shoulders and fix the problem, or figure out what went wrong in the first place to try and prevent it from coming back again. While I don't always have the time or the smarts to suss out the root cause of every Mac issue, sometimes the issue is so curious and the cause so interesting that I feel compelled to investigate until I get to the bottom of it.
Recently, when one of my colleagues came in with a slightly-full hard drive, I went to my go-to disk space checker (the capable OmniDiskSweeper) to see where we could save a gigabyte or two. In addition to the usual suspects of iTunes podcasts long gone stale and legacy backups of Entourage databases, I came across the file you see above; it lives in ~/Library/Application Support/Chess. Why on earth would a support file for Chess.app be 1.5 gigabytes? That's crazy talk. I would love to delete it, but a file that large... might be useful or important.
Where did this bulky bucket of bits come from? Read on for the answer.
Since the .lrn sounded like it might be a "learning" file of previous chess games, I asked the user if he played chess on his Mac, and he said no. I searched for "standard.lrn large" and found no major clues, although I did learn that Apple apparently based Mac OS X's Chess.app on the freeware Sjeng engine. I decided to sleep on it... and the next morning, I had figured it out.
I remembered that on a previous machine, this user had experienced some odd issues that we suspected were hardware-related. In order to do a bit of a torture test on the processor, my IT colleagues and I decided to do overnight CPU exercises, by having Chess.app play itself for hours on end. Hence the massive .lrn file, which had carried over in the user folder through a machine migration. File deleted, mystery solved, apologies extended, user happy, and another case tucked away in the CSI: Mac archives.