sleeper cellで試しにググッたら、ISがらみで頻繁に使われてるね。
日本の外では、当たり前に実在してると認識されてる。

Russian security service says dismantles Islamic State sleeper cell　　
OCTOBER 2, 2017
https://www.reuters.com/article/us-russia-plot-detentions/russian-security-service-says-dismantles-islamic-state-sleeper-cell-idUSKCN1C7167

France tracking 400 believed to be in terror sleeper cells
February 18, 2015
https://nypost.com/2015/02/18/france-tracking-400-believed-to-be-in-terror-sleeper-cells/

Sudanese immigrant wanted to start a ‘sleeper cell’ for Islamic State
February 23, 2017
https://www.washingtonpost.com/local/public-safety/sudanese-immigrant-wanted-to-start-a-sleeper-cell-for-islamic-state/2017/02/23/ecaa0c36-f9e9-11e6-be05-1a3817ac21a5_story.html?utm_term=.8065abbaa711


Islamic State does have sleeper cells disguised as refugees
Aug 11, 2016
http://www.washingtonexaminer.com/islamic-state-does-have-sleeper-cells-disguised-as-refugees/article/2599122