The trusty flare gun you get from Preston can be a way to spawn more gaurds in your settlement as they tend to stick around from what I've seen.. I've happened to stumble upon this great thing by accident. I was fighting some mobs and boom come a legendary blood bug so I set the flare and fought off the creatures after a little while I killed it, they showed up late. Then I was like ok see ya guys later.. And I did.. At one of the smaller settlements just hanging around with laser muskets.. So take this tip and try not to spread it to much that it'll get patched.. Have fun peeplees

So to be clear, they were in the same place you shot your flare right?

No they were somwhere else and they went to one of the settlements I recruited.. Though it seems the bug is bugged I tried testing it out near red rocket to help fortify it but they dissapear when I fast travel.. Still wonder why I have one of em at that place though.. Oh well I thought it would be cool but I guess they leave.. Or maybe when I launch the flare and they arrive I need to load a save which hopefully resets their AI.. Idk it worked once unexpectedly.. Not like it matters as we will have the GECK in months time

While I was walking I saw them protecting a bridge. I just wonder if they start spreading out more as you build them up through their missions