Green’s Dictionary of Slang

a trophy; something that has been found; thus a euph. for something stolen.

1919		W.H. Downing Digger Dialects 46: souvenir (n.) — (1) A trophy; (2) something found; (3) euphemistic term for stolen goods.
1920		Bulletin (Sydney) 19 Feb. 14/2: Conk, the Souvie King of our battalion, got most of his pickings across [i.e. from France to Australia] that way.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 264: Souvenir: A trophy. Something to take home from the Front.
1941		A. Gurney Bluey & Curley 5 July [synd. cartoon strip] Find out how that fellow Bluey gets all those souvenirs from the prisoners.
2000		Ozwords Apr. 2: It is in First World War Australian military contexts that souvenir in the sense ‘to appropriate; to steal; to take as a souvenir’ first appears.