You've probably heard the old adage that says, "good fences make good neighbors." It comes from a poem by Robert Frost, and it's good advice if you ever want to sell your property. Unmarked boundary lines can result in your neighbor building either all or part of a new garage on your land.
You might not have a problem with this at first since it doesn't interfere with how you live on your property. However, if you put your property on the market, a potential buyer could take issue with the garage's position. For this reason, if you even slightly believe that you could sell, you may want to address the issue now rather than later. Of course, the best time to do so would be before the structure goes up, but you may not have fully considered future problems when it happened.