Additive packages are sophisticated enough now that testing of additized gasoline can tell whose pahckage it is, or maybe more importantly, isn't.
You've maybe seen the Exxon commercials where they refer to marker moleculss as the final ingredient? That type technology has been in play for well over a decade now, Exxon commercials are the first I've heard publicly advertise it (but not really go into depth on the subject of any of the ingredients of course). It's more than just for determining additive package concentration.
http://synergy.exxon.com/
I suspect that ExxonMobil doesn't make their additive. There's nothing on the EPA approved list of certified detergent additives from ExxonMobil or Infineum. It wouldn't surprise me if it was actually developed as a proprietary additive by one of the large chemical companies, like how Costco had Lubrizol develop Lubrizol 9888 as their own proprietary additive. The BP Amoco additive is listed as being made by Innospec. However, I suppose that doesn't preclude them from using the additive as a "tracer" as you suggest.
https://www3.epa.gov/otaq/fuels1/ffars/web-detrg.htm
What I'm saying is some companies have used different "marker molecules" in their additive packages for years, regardless of who makes the additive package, to identify fuel spiked with their brand's additive package. AFAIK there's no legislative requirement for this, it's something each company /branding can decide on.
I know Diamond Shamrock, who Valero got their first entry to retail through, did not. However treat rates for different customers were in fact different. Now it's 15 or more years down the road. Now Valero has sold all their retail originally to Corner Stores, now they're owned by Couche Tarde, and Valero's CEO complains on how RINs are applied with no company owned retail arm. Here in San Antonio, the home HQ for Valero, there are some Shell corner stores and probably a few other brands. As Valero is now back to being a pure merchant refiner, I see no incentive for there to be a Valero additive package with Valero specific "marker molecules". Just meetint Top Tier should be sufficient.
Costco wouldn't need "marker molecules" since they additize solely at point of sale.