I've had one for quite some time and didn't know it until a doctor (a chiropracter) looked carefully when I arched my back enough and could see it. I think I got it from years of heavy abs training and it sometimes really hurts if I train my abs directly. But, my doctors (internal medicine and GI doc) say it's "small" and are not recommending surgery. I'm thinking of finding an appropriate surgeon who is willing to fix it because the pain, when it happens, doesn't feel "small" at all. It can be pretty serious when doing core exercises, especially abs.
I used to train abs directly and thoroughly with no problem at all, but the hernia pain gradually became evident.