'Likely' triggered a red flag? Interdasting... what info do you have that leads you to say that? You know they don't have a comprehensive bad brown person list, right? And what are they going to find when there's extra scrutiny, a man with nothing remarkable on him? (There are some reports of a gun, but if he'd had to go through border checks he could have not bothered, or bought one there). France wouldn't be closed to tourists, with it being fairly easy to create a convincing explanation. Then it's pretty easy to move around without being caught... hop in the back of a HGV wherever and you can get anywhere in continental Europe.
There a lot of flags someone can trigger crossing a border. A single ticket, no address in the country, no return date, passport, criminal record, already flagged by security agencies amongst a few. A border control would have stopped many of the attacks in France. Tourists don't create flags. Immigration control staff are extremely capable at spotting people that aren't tourists.
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