This is the terminal at KSTS. Someone mentioned to me that your can't overfly this area. (I fly helicopters direct to the ramp adjacent.) What regulations discuss this? I don't see it in the AIM.
I believe it's a non-standard marking to indicate a Security Identification Display Area (SIDA), where you need a specific badge to have access. The TSA sets the requirements for access. If you zoom in you can see the words "AIRLINE EMPLOYEES ONLY" beside the markings.
I don't think there's any standard marking for SIDAs. There was a proposal in the draft version of AC 150/5340-1K - Standards for Airport Markings (see section 5.15 for all the details):
The SIDA marking is a rectangular stripe with white legends that are centered within a red background. Paint colors are per AC 150/5370-2, Item P-620, Runway and Taxiway Painting.
But, it didn't make it into the final version:
Drops a proposed new surface marking for areas designated by the Transportation Security Administration (TSA) and the airport operator as a security identification display area (SIDA) (reassigned paragraph 5.15 to cover markings for aircraft deicing facilities and removed proposed figure C-14). TSA accepted the task to address future markings for the airside.
And as ymb1 pointed out in a comment, it's not in the current version of that AC either. Presumably the TSA is still working on it.
If you look at a SIDA at another airport, you can see differences. The SIDA at KLEX (Lexington, KY) is marked by a single red line, for example: