It's just the properties of glass that make them give you that queezy feeling in the pit of your stomach. Even if you knew exactly how it happened, there's just no way to tell if it will cause an issue at some point.
I have 26 tanks running at the moment, and 9 of them came to me used. The biggest is an Oceanic 180G. I have a 29G that must've been 20 years old when I got it (O'Dell) and the silicone looks like somebody took a razor blade to it, and it's been just fine for three years now. Then there's the 90G I bought from someone and a few years after I filled it I saw the bubbles in the silicone seam. That was about 10 years ago. No issues there either. I also crazy-glued the brace on that one. If you read enough stories you'll read about the leakers right of the box.
In short, used tanks don't come with guarantees and even if a new tank leaks, they won't do anything more than replace the tank.
Fill 'er up and let it stand for a week. You'll have your answer. Just pay some special attention to the silicone behind the chip and directly above and below it. If you can see small air pockets in that area, it may have been whacked pretty good and even I would pass on it at that point.
"Why can't my wife see all of this stuff as an investment?"