Epsilon IX was the station that fell victim (after the Klingons) to V'Ger in Star Trek: The Motion Picture. Officially, Epsilon was a communications station and transceiver for subspace messages, but it's position near the Klingon border makes you wonder if maybe it wasn't something more. Why would Starfleet need a manned station near the border of a then-current enemy if all it was built for was to handle communication traffic? And why put one there? The fact that they were able to observe the destruction of the Klingon ships that attacked V'Ger would imply otherwise. Certainly makes you wonder which division was really operating Epsilon - Communications or Intelligence?
The models of the station and panel were later reused as a wall decorations in the Officer's Lounge in The Search for Spock. After that, the models disappeared without a trace because that's something that can somehow happen.