I can think of one particularly famous SpaceX 2nd stage in heliocentric orbit that even has its own website: https://www.whereisroadster.com/ and I know one was used to send DSCOVR towards Sun-Earth L1, though I'm not sure how it got there:
- Did DSCOVR travel “along the stable manifold of it's future SE L1 Halo orbit” to get there?
- Can Lissajous orbits have stable/unstable manifolds?
But it's the SpaceNews.com article about an upcoming launch NASA awards DART launch contract to SpaceX got me thinking, how many Falcon 9 2nd stages are "out there" beyond Earth orbit?