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?
Best Answer
This happens. I've had an instance where there were definitely no enemies and in that instance you're just going to have to deal with it.
Essentially, explore the entire region around the settlement and see if you can find more enemies, if you can't, sit around for a bit and see what happens. If you have a siren then use it, see if the call to arms allows one of your settlers to see the enemies that are supposed to be attacking.
Settlement attacks are random rather than pre-determined which is why reloading the save didn't allow you to relive the attack.
Additionally, settler count and settlement statistics via the Pip Boy interface are not always correct, as discussed here in this question.