I've just been informed that a settlement of mine — Sunshine Tidings Co-op — needed defending, so I went there and witnessed my turrets make short work of 3 of my settlers; it turns out they were synths.
With the synths dead and the problem solved, I decided it was time for me to move on. Only, after fast-travelling, I received a notice that I had failed to defend the settlement. And now, according to my Pip-Boy, Sunshine Tidings Co-op only has 3 settlers instead of the 14 I had left it with.
So I decided to load a previous save game and wait for the actual attack to happen. I kept waiting one hour at a time for about 24 hours, and the attack never came. So far, whenever I came to defend a settlement, I either ended up right in the middle of the assault, or the attackers appeared after a few seconds following my arrival.
I don't believe I can actually defend the settlement this time. But why not? Am I supposed to discover another synth in hiding? Should I build even more turrets there? Is there anything I can do to avoid losing almost all of my settlers?