I believe "see it coming" is mean in the way that the security services would be able to predict the attack precisely (when, where, who), given the amount of survelience, data gathering and spying they do.
Alas, the spying is apparently targeting mainly the innocent civilians and is mostly ineffective against criminals.
If the NSA revelations are anything to go by, the only difference between "not enough data" and "too much data" is that the after-the-fact investigations are now a bit easier.
There doesn't seem to be a "Goldilocks" amount of data. Or if there is, intelligence services seem to have no interest in finding it if it means restraining themselves.
From 5 days ago, Travel Stack Exchange, "What areas to avoid in Brussels after the recent terrorism events": https://travel.stackexchange.com/questions/65332/what-areas-...
Sadly, there were really no answers, just the usual meta discussion.
For any city, I could give the advice: avoid the airport, trains stations, the metro, and famous tourist attractions.
On the other hand, these are typically the places that you need and want to go if you visit a city, so it's anyway better just not care. The risk is very, very, very small.
But I agree that this is a problem we can't bomb our way out of.