I had the impression there are some "secure areas" at various Thalys stations, which I don't think have ever been used.
In Brussels, they sometimes do spot-checks, although I wonder how often that still is. I've run into it only once. They've converted one room close to the Eurostar terminal to be able to do luggage checks there.
In Liège, they've put an ugly container-like box on the eastern overpass that I've never seen used. I wonder what Calatrava would think about it. In Antwerp, I'm not sure if and where they hid the luggage checking equipment. In both Liège and Antwerp, crowd control barriers have been put on some platforms (2/3 in Liège, 21/22 and 23/24 in Antwerp), meant to separate passengers somehow if there are checks, but I've never seen them used and they're basically just an annoyance for passengers.
In Paris Nord, there was some luggage checking equipment installed at the beginning of the Thalys platforms, but I've never seen it used and IIRC it has been removed quietly.
In the Netherlands and Germany, nothing visible to passengers was done.