Green’s Dictionary of Slang

(UK Und.) a piece of paper that is left to mark a door for the purpose of robbing an empty house; also as v.; thus trigging the jigger n., the placing of such a marker.

1777		J. Fielding Thieving Detected 10: They go and trig the street door.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 276: trig: a bit of stick, paper, &c., placed by thieves in the keyhole of, or elsewhere about, the door of a house, which they suspect to be uninhabited; if the trig remains unmoved the following day, it is a proof that no person sleeps in the house, on which the gang enter it the ensuing night upon the screw, and frequently meet with a good booty, such as beds, carpets, &c., the family being probably out of town. This operation is called trigging the jigger.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.