Ant remediation and property lines

The husband has handled the ant problem. He sprayed, and poisoned, and put out death bait. I'm sure it helps that it got cold again, too. Knock on wood - there have been no ants in the house this week. This is extraordinarily exciting for me, and if it keeps up, I can stop obsessively searching the bed every night and my socks every morning. Last week, there was a day of winged ant infestation in my kitchen window. I made the husband handle it, which he did. Because it was so gross.

Then we discovered a problem of a different nature - we found out our property line is different than we thought it was. Our neighbors apparently put up a fence inside their property line. I am not sure why they would do this. There is no gate, or anything, just a fence, and then a strip of land a few feet across, and then our property apparently starts. We had just been assuming that the property line started at the fence - but instead it starts at this decades-old retaining wall, which clearly runs between two metal posts. The posts are actually mentioned in the deed. Obviously we failed to do our due-diligence here (not that it would have mattered to us at the time of purchase, but still a failure). It's a bit disappointing to find out that you don't own a strip of land, however small, that you thought was yours.