Although I'm not actually sure how much the watchdog is "warning" here, despite the article claiming it is. The language from the report is the following:

    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.

The report doesn't make any recommendations or suggest any changes to the law, it seems to consider that it is good that this could be considered Section 3 "hostile activity". This later part about Schedule 3 investigations also makes this clear:

    The Joint Committee on Human Rights was of the view that there was an even greater risk of arbitrary use of Schedule 3 than of Schedule 7, owing to the broader and more ambiguous definition of “hostile activity” (than of terrorism). I would develop the point further:

    • For Schedule 7, the examining officer must at least have in mind the possibility that the examined person is morally blameworthy. That arises from the statutory question: whether the individual is a person “…is or has been concerned in the commission, preparation or instigation of acts of terrorism”
    
    • By contrast, under Schedule 3 a lawful examination may take place, with all the intrusion and inconvenience attached, of an individual for whom the question of moral fault does not arise at all. 

    6.22. Having said this, I am not yet able to draw any practical conclusions from this conceptual distinction. Firstly, most examinations are likely to be directed at the issue of witting or witting participation, which incidentally puts a premium on human as well as digital interrogation. Secondly, the possibility that an examinee is in fact entirely innocent also arises under Schedule 7. I will therefore keep under review whether this needs to be reflected in the Code of Practice, but make no recommendation at this stage.