up to something

I took my mare and went down to the swamp, for I knew they had been up to something there from the signs I saw.

1871 October 27, Prince McElhannon (witness), “The Ku-Klux Conspiracy”, in Testimony Taken by the Joint Select Committee to Inquire into the Condition of Affairs in the Late Insurrectionary States. Georgia, volume II, Washington, D.C.: Government Printing Office, published 1872, →OCLC, page 649

One or another of them will be poking their heads in and see we have been up to something [...]

1922 February, L. B. Thoburn-Clarke, “Amid the Swamps. A True Story of New Zealand Life in the Early Days.”, in The Boy's Own Paper, volume XXXVIII, part 4, London: "Boy's Own Paper" Office, […], →OCLC, section II, page 212, column 2

One day Chander, who was always up to something, took it into his head to ring up one or two customers in regard to bills long outstanding on them.

1955 January 22, S. Roberts, “The Man who Made the Grade: A Short Story”, in Ram Singh, A. K. Mukerji, editors, Thought, air edition, volume VII, number 4, New Delhi: Printed on behalf of Siddhartha Publications Ltd., by R. L. Chadha at Naya Hindustan Press, →ISSN, →OCLC, page 10, column 1

If they are doing nothing they are going to be up to something – just send them to prison.

2005 August 25, Simon Cox, “A 'Neet' Solution”, in BBC News, archived from the original on 08 Jul 2018

Shepard: Last I knew, Kaidan was Alliance. Why is he out in the Terminus Systems? Illusive Man: Officially, it's an outreach program to improve Alliance relations with the colonies. Illusive Man: But they're up to something. And if they sent Commander Alenko, it must be big.

2010, BioWare, Mass Effect 2 (Science Fiction), Redwood City: Electronic Arts, →OCLC, PC