This answer to Why would a mission to Sun-Earth L1 have an instantaneous launch window? suggests that this was quite a delta-v-intensive launch. It left the F9 2nd stage in a very high 17 day orbit around the Earth. DSCOVR is now in a heliocentric orbit in resonance with Earth, otherwise known as a Halo orbit or in this case a Lissajous orbit around the Sun-Earth L1 point.
In Why can't custom-made TLEs for the DSCOVR launch booster in orbit around Earth work with SDP4? I estimate the period of the first stage rocket body to be about 17 days based on TLEs in links in this answer. That makes the semimajor axis roughly 280,000 km, which means it's subject to strong perturbations from the Sun and Moon. That linked answer also says:
It is true that TLEs for 2015-007B = DSCOVR booster are not provided by Space-Track. However, that object (and others in high Earth orbits) are occasional annoyances for the astronomers looking for near-earth asteroids, and they've imaged 2015-007B many times over the last four years. From these data, I've computed orbital elements and fitted TLEs :
which means some precise sightings have occurred. However, spent rocket bodies are large, white and mostly hollow and light, so they are subject to significant photon pressure and other non-Newtonian forces. For more on that see
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Question: Is it known or has it at least been hypothesized where the DSCOVR mission Falcon 9 2nd stage will end up? Will it escape to a heliocentric orbit, or eventually crash into the Moon, or does "where it stops, nobody knows" apply?