The Last Ship?
I was thinking of the infectious disease flag that ships raise to warn others that they have not been certified by the local port authority as being safe. Formerly, the all-yellow flag “Q” was used to indicate a quarantine (”yellow jack” for yellow fever, giving that disease that particular nickname). The “Q” flag is now used to indicate a request for the port authority to come aboard and confirm that all is well. The yellow and black flag “L” is now used to indicate a current quarantine.