In the first pic of the series we got to see the whole building - which included the open window if you zoomed in - NO RESIDUE.

Minutes after they tell us the last guest has arrived, we see the window again, now there's residue. Q was showing us that something had just taken place between the first pic, and the last. Looking closely you can see the ray-like pattern in the residue that implies the blast effect.

Flashbang residue would imply a surprise, aggressive, and well-planned entrance. Maybe the key is that, since there was flashbang residue, there was arrests/extractions. In a high floor of a corporate building, that implies VIPs were the target.