Green’s Dictionary of Slang

(US) a plain-clothes police officer, a detective.

1944		C.S. Montanye ‘Death with Music’ in Thrilling Detective Feb. 🌐 Mullin, Hartley and some plainies tumbled out and the prowl car whined off.
2011		J.J. Connolly Viva La Madness 37: Half the dudes knocking around this marina are plainies [...] or registered informants.