Green’s Dictionary of Slang

(UK Und.) posing as a door-to-door salesman to tour houses and thus identify empty ones, ripe for robbery.

1925		N. Lucas Autobiog. of a Thief 105: Crooks go ‘drumming’ in pairs, dressed as clerks or messengers. Towards lunch-time they proceed to a large office building, select the name of a firm which they know to be located on the top floor, and visit a number of offices [...] at which thy enquire for the selected firm. Should they find an office left unoccupied [...] they quickly and skilfully ‘turn it over.’.
1936		J. Curtis Gilt Kid 101: He made a resolution to come up this neighbourhood, drumming some time and see if he could not pick out a nice little chat to screw.
1977		D. Powis Signs of Crime 182: Drumming Calling at dwellings to see if temporarily unoccupied so as to facilitate breaking and entering Sometimes used to describe honest door-to-door salesmanship.
1989		J. Morton Lowspeak.
1999		Indep. Rev. 19 Aug. 13: Ray, who did a three-year stretch [...] has relapsed into ‘drumming’.