My guess is he exhibited behaviors that were not normal to tourists. Kind of like people who are not from NY who look and point at the buildings.
The day after the Paris massacres in November, we saw heavily garbed Muslim women with infants in strollers more or less casing the local mall and coffee shops. They showed no interest in the merchandise in the book store that adjoins the coffee shop where I was sitting. And I have not seen them again. It certainly seemed like a surveillance visit.
I can also recall noticing people of Mid-east appearance taking photos on the car deck of a large ferry shortly after the 9-11 attacks. There again, no rational reason for taking photos other than surveillance.
Personally, I suspect that this is organized and widespread, that thousands of targets are already cased and reported or at least available if the right people ask. I don’t think they have the resources to attack all of them, and that they are waiting to attack a few when it suits their purposes.
And this was in Canada, I can only imagine that the interest in doing harm in your country is that much greater.