I found out that my 85 year old house is partly on someone else's empty lot.

I purchased my 85 year old house in 2001.
They purchased their property in 1975 after my house was built.

They recently had their property surveyed and found that part of my garage is on their property. I took them 35 years to find out that part of my house was on their property. So far, they have not approached me with any demands.

What are my rights?

I live in Pierce County, WA