I found it:

https://www.gov.uk/government/publications/report-of-the-independent-reviewer-of-state-threats-legislation/report-of-the-independent-reviewer-of-state-threats-legislation-accessible

It’s an independent review of some UK laws concerning national security, and the reviewer is warning that the laws could be used against people unfairly. Note the last sentence of the section: “Serious responsibility is put on police to use the power wisely.”

Engagement in Hostile Activity

6.16. Under Schedule 3 a person may be engaged in hostile activity even though unaware that their activity is hostile activity[footnote 428].

So a person could be examined on account of their wholly inadvertent and morally blameless conduct.

Examples could include a journalist carrying confidential information whose significance to national security he did not understand, or the victim of planted material. The examining officer could act if there was no possibility that the person was aware that its dissemination might be in the interests of a foreign state, or even that they were carrying the material.

The Code of Practice to Schedule 3 refers to the innocent dupe, who “…may believe that they are working for a legitimate business, or charity, which is in fact being utilised specifically for the purpose of espionage”[footnote 429].

6.17. Since hostile activity does not require any knowledge or tasking by a foreign state[footnote 430], the phenomenon of double-ignorance could arise. A person may be engaged in hostile activity if they do something which, unknown to them threatens, national security and which is in the interests of another State, also entirely in the dark. For example:

The developer of an app, whose selling point is end-to-end encryption which would make it more difficult for UK security and intelligence agencies to monitor communications. It is a reasonable assumption that this would be in the interests of a foreign state even if though the foreign state has never contemplated this potential advantage.

The lobbyist for a foreign firm, who seeks to persuade an electronic chip manufacturer to build its factory in France rather than the UK. This would engage the UK’s economic well-being in a way relevant to national security even though France is entirely unaware of the lobbying and the lobbyist is only doing his normal day job.

A journalist carrying information that is personally embarrassing to the Prime Minister on the eve of an important treaty negotiations affecting UK security interests.

6.18. In each of these cases the motive of the app developer/ lobbyist/ journalist may be more sinister than first appears, so permitting an officer to examine whether the individual is a witting or unwitting agent of a foreign state might be described as necessary in the right circumstances. Serious responsibility is placed on police to use the power wisely.