>The reasonable thing to do with that information, would be to surveil them further, search their house, or arrest them.
That is in no way a reasonable thing to do if you're only 50% confident in your results. A reasonable thing to do would be scrap the whole system and make something more reliable than a coin flip.
Characterizing it as a coin flip is unfair I think. If this system can promote individuals to attention out of millions of targets, telling us that this person out of all the millions has a 50/50 chance of being a terrorist, then that is hugely valuable information that deserves further inquiry.
Now, of course we don't know how good the rate is or what other evidence they already have or what exactly a terrorist is for that matter. But in principle, you'd be throwing out a huge amount of evidence if got rid of a system that changed a target's chance from 1 in a million to 1 in 2.
That is in no way a reasonable thing to do if you're only 50% confident in your results. A reasonable thing to do would be scrap the whole system and make something more reliable than a coin flip.