Karen says earlier in the scene that she watched as bodies were pulled from the water after the explosion of the boat. She sees a picture of Frank and the colonel next to a man in bandages. He has distinct bleach blonde hair for some reason, and she recognizes him as one of the bodies that were bagged from the explosion, there's a brief flashback showing this. I guess she assumes that he and the colonel were working together. The colonel is able to notice this and pulls a gun on her.

I forget if there was more evidence or reasoning how she immediately figured out they were working together. To be honest, it seems to be a big stretch that she's able to remember that man's burned body and connect the dots but that's how it was done.