What sort of place where you in where you found the symbol? Private office, public restuarant, local bus station?
If it’s a highly public place, try asking the management. They may have a clue. And management may not be aware of gang signs in their lavs; they may be glad to get the news so they can clean it up.
If the neighborhood is such that gang territorial marking is possible, call your local police public realtions dept and ask; they’ll certainly have a glossary of the local symbology they’re up against.
Other than that, I don’t have any suggestions