Hi Damon and welcome to the forum.
You say... "... but in my factory, people tend to bring in their own supplies on occasion, and then these supplies vanish. ..." And... ".... Other than posting signs telling these people to make sure they don't bring their own materials... "
When you say these things, I expect you're talking about low dollar things like PB Blaster, WD40, Anti-Sieze, Loctite, etc. that might help their day go more smoothly... and not some high dollar crazy juice?
If thier bringing in their own materials then your employer is pretty lucky I'd say. One less thing to pay for you know... But if it's a problem then you tell them to stop it. If they continue then you fire them. If it isn't a big deal then you look up the MSDA's for the stuff they're bringing in and post it. Just because something is in your MSDS book doesn't mean it needs to be currently in stock. Stock meaning in house by whoever's means. Doesn't matter if you haven't had it in house for years... it can still be in your MSDS pack.
If the materials they bring in contain certain undesireable chemicals then you might need to simply put a stop to it. Some businesses forbid the use of certain chemicals in the production of their products even at a supplier level.