As you can see in the picture above, there used to be a plaque on that brick wall at AT&T Park.
It was a plaque commemorating Barry Bonds' record-breaking 756th home run.
The SF Chronicle has a somewhat interesting theory that perhaps the GIants are trying to erase all memory of Bonds, who was one of the greatest players in Giants history, yet hasn't been honored as such by the team.
His number hasn't been retired, he's no on their Wall of Fame, and he's not in their statue garden. Despite the fact that he was arguably a better player than anyone else in team history, except for Willie Mays.
It turns out that this plaque was stolen, and it's not just the Giants trying to hide all traces of Bonds' memory. They're making a new one right now, and it'll be replaced soon.
However, it sure does seem like an inside job. It's a 6-feet sign, and a fan would have to pry it off the wall right in front of thousands of other fans while the game was being televised, and then they'd have to somehow sneak it out of the stadium.
An employee, on the other hand, could enter the park any time, steal the sign under the cover of darkness, and then walk right out of the park, or even drive it out, right in the back of a pickup truck.
We'll see if they end up finding the culprit, but I tend to doubt it. The fact that they've already announced that they're making a new one seems to indicate that they're not terribly concerned with finding the original.
Maybe we'll see it on eBay someday.