I only find weapon schematics at the irregular markers, two or three containers, threat 1, 2 or 3 - no matter. After raiding ALOT of those I have stocked up all I need to engineer my weapons, but no engineer to go to.I had one Settlement Defence Plan; I made the mistake of trying to hand it in to him a couple of days ago, resulting in it vanishing from my storage yet not apparently decreasing the number he wants. Beware of trying to hand in fewer than all 15 at once.
As far as finding them goes, so far it seems they are only obtained from the Threat 1 Irregular Markers smuggler caches with two walk-in cargo containers and a crashed skimmer, downloadable from the data port on the skimmer (and only sometimes). The three-container variant has a SRV instead of a skimmer, so that's no good. They do not appear to be obtainable from settlements, contrary to all common sense - unless someone knows better?