Exactly. If the thing you are trying to predict is very rare (compared to business as usual), the error rates of your system have to be unfathomably low if you don't want to either drown in false positives or detect nothing at all ( =expensive money wasting project).
In some cases the error rates need to be so low, that even claims like "we are 99.9% accurate" should be taken with a big crystalline block of salt.
Remember this whenever terrorism is used as a reason why we desparetely need these surveillance mechanisms. If burglars are rare, terrorists are even rarer. The proposed systems are good for bigger things like demonstrations, social movements etc. but they suck at detecting terrorism, unless you have a metric ton of personal which will look at false positives all day long.