I have this mission hanging in my hud since 2016 and never found any answer over the internet how to fulfill the requirements. Only appeared to this toon that is Romulan KDF allied.
The mission is just a wee bit outdated now. There used to be enemy signal contacts roaming about that you could engage to complete the mission. The mission giver was in Battle Group Omega and was applicable before the sector space boundaries came down. Each sector block had it's own red alerts and deep space encounters. Now it's an STO wiki stub.
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https://sto.fandom.com/wiki/Mission:_Defend_the_Beta_Quadrant