Seriously, though, this should be a thing. Every vendor should have a place they are stashing their stuff, kind of like how you could steal inventory from vendors in Rivet City in FO3
I have a few other ideas you could add to this fantastic mod suggestion. First idea Settlers need 8 hours of sleep in order to work at full capacity and can work for up to 8 hours a day without penalties. When setting up what a settler will do in the management UI also have a place to set up their shift schedule. That way you need a constant rolling supply of guards for 24/7 defense. This would also allow you to schedule meal times and recreational times so you don't have entertainers entertaining an empty room or people eating in the mess hall while the chef is sleeping (for immersion's sake). I think another nice addition would be an option to access a settler's inventory through the management UI and add/subtract things through your settlement inventory. This makes the UI a complete one-stop-shop so the moment you assign a settler to a job they look the part and have a full daily routine planned out for them. Second idea If happiness drops below a certain threshold (say 40%) then lower level settlers have the option of defecting to raiders around the area. This will in turn give the raiders small bonuses when attacking you because they have inside information about your settlement.