Every day I have about 15 new flares in my sanctuary workstation. Does anybody know what resource is creating these?

Probably Preston putting them in there since he gave u the original flare gun

Very likely, as I think it's only in the Sanctuary workstation (or perhaps whichever settlement Preston is at --- I left him there). If you set up a Salvage station in a settlement, and assign a settler to it, you'll get a wide assortment of random crap, but mostly (only?) junk for scrapping. If you have surplus food and water, those go into the local workstation, too, along with profits from stores you set up. Not sure if non-flare ammo ever gets added....

Thanks for the replies. I'm thinking they're probably from Preston as well. So far flares are all I've seen in the ammo section, I do get a ton of random junk from the scavenging stations though.