bandit sign
n. An illegal commercial sign posted in a public area.
Houston City Councilman-at-large Gordon Quan led a team that removed "bandit signs" from the esplanades and roadsides of the streets. "The signs on the stakes are illegal, so just pull them out ," said Quan who said people placing such signs are subject to fines of $ 150.
—Charles de Magin, “Alief weekend cleanup fills 730 bags,” The Houston Chronicle, April 04, 2002
1987 (earliest)
Each month the company spends $ 300 putting up bandit signs at a cost of $ 6 apiece for its projects along roads leading to the competitions' developments — big builders who advertise heavily and generate enviable traffic. And as the big builders use their ad dollars to draw buyers, Palmer Homes' signs along the way piggyback on those drivebys. Surveys show the signs generate half of his traffic.
—Joe Catalano, “Small builders, big ideas,” The Washington Post, July 17, 1987