Just wanted to say that I've been a big fan of this forum and have been lurking on it for a while. I've read lots of really good and insightful discussions about TWD and finally got the urge to sign up when I read this post, so I wanted to offer my own speculations based on what I've seen on the show and read on the forum so far about Rick's gun. Here goes nothing...
There are only 4 main people who witnessed the initial pistol exchange:
We can eliminate Shane and Otis since one is the culprit and the other is dead.
That leaves us with Rick and Hershel. Both Rick and Hershel:
1. Witnessed the pistol being passed to Otis before the medical supply run to the FEMA shelter
2. Heard Shane's suspicious eulogy about being "down to pistols" in the zombie gun battle
3. Witnessed Rick's pistol placed on top of the car's hood when Hershel announced his "no firearms" rule.
Since Rick isn't all keen on the fact that he got his gun back, that really only leaves Hershel for being the first person to piece the puzzle together (as it was discussed in this forum that Dale was not present when the pistol was originally exchanged).
Here's my (speculated) spoiler synopsis:
Any thoughts, or am I way out of the ball park?