The message is attributed to Sol. If it isn't a sol gate them I'm lost.
There are five gated jump point from Sol in this campaign. Two or three have smi-permanent pickets by at least one faction. Most factions are on passive sensors only, but the faction receiving the unknown transit warning has active sensors running at one location. Unless I'm missing something, the active sensors are the only thing that makes this faction different from the others.
There are civilians using jump gates, but not the particular gate with the active sensor picket. I think. I'd also think any transit message related to a transponder would also be seen by multiple factions and not just the one.
I'd say it sort of defeats the purpose of a picket if something can transit without you knowing which gate, what direction, or who.