You understand how bayesian reasoning works, right? There are probably between three and six
million muslims in France alone. Whatever monitoring you're doing, you're proposing to find on the order of a few hundred people, if that. False positives (which, remember: we cannot distinguish from actual positives) would end up targeting orders of magnitude more innocent people than guilty.
Worse, by breaking the compact of a free and just society with respect to a single religious group (cough), you're basically doing the terrorists' jobs for them - you end up marginalizing and disenfranchising huge swaths of your population (remember again: millions of people in France alone). Even if one is not not ethically and morally appalled by your proposal, the problem remains: your proposal would probably contribute more to radicalization than any other single action the terrorists themselves could take.
It is at a time like this when we most need to stick to the principles of a free, just, and open society. We need to re-affirm those, and I say that as an American who has lived through the post-911 transition into a security society. Learn from our mistakes: we've gone and made people more radical, spent more civilian and military lives, because as a nation we somehow thought we could fix the "muslim problem" through fighting and surveillance. Neither worked.
What will (eventually) work is working toward a nation and a world where all people are free and equal. Sure, there may still be attacks against such a nation, but that's what police are for (when the police are also just and equal). Furthermore, it's a lot harder to radicalize people when they live in a safe and just society that pays attention to them. I'm not surprised at all, given how non-muslim westerners have treated the religion and people, that we are seeing radicalized people.
And aaaaalso, this bears mentioning although it's kind of tangential. Not all muslims are brown. Not all muslims speak bad english / french. Consider all the work Imperial Japan did to wipe out christianity (you can't tell a religion by looking at someone), and all they did was make it stronger and drive it underground.
Consider: in the US, actually, the biggest terrorist threats come from white people. White people of the white-supremacy and militant-anti-abortion camps are responsible for far more terrorist attacks on US soil than ISIS and al-qaeda combined. We, as a nation, just don't really like to talk about surveiling all white people. It'd be ridiculous, a fact we understand instinctively because the white narrative is the non-marginalized one.