Confrontation of course! Well, on the long term.

I was thinking of two possibilities.

1: The mild one. My peeps getting more and more involved in galactic affair, reveal their use of technologies that are a copy of those used by your peeps, proving that they got hold of at least one colony of theirs and salvaged everything they could find, probably killing or capturing everyone inside. Alternatively since your race is one of powerful psions, one captured individual sends a distress signal with his mind asking for someone to save him.

2: The fun one. Out of nowhere, ships come to your current world with an IFF marking them as yours, despite the design being completely different. They are quickly dealt with but a frightening thought occures: The automated systems would have allowed these intruders to land and enter your sanctum with no problem whatsoever. After investigating, you find several colony worlds that have been opened and completely ransacked. Someone found how to fake your Identification Friend or Foe and a map of the colonies, and is now methodically cracking them open one by one.