Your instincts are good, and scams are out there, but there's a possibility that it's something legitimate that you don't recognize.
Twice now, I've been really startled by PayPal deposits from companies that I "knew" I hadn't done anything for. (Why is White Clay sending me money? Well, the company that I worked for decided that it wanted a better name. Why is HS Research sending me money? Well, that's an alternative name for the company, and I'd forgotten it.) So, if those had been checks, I'd have wanted to cash them.
This will only be good for the future, but make sure that you keep a list of all your shops: company, client, amount, date, and all the rest. Check the items off as your're paid for them. Then, if you get something mysterious, you should be able to figure out if there's anything it could be. (Google the name of any possible MSCs, along with "Access Research", to see if there's any connection.)