Colin Kaepernick flew into Atlanta on Thursday to get ready for his NFL workout -- and our sources say he's already in game mode.

Kap touched down around 5 AM in hopes of getting through the airport without fanfare -- and he pretty much did.

We're told he was with a very small group of people and the person who shot the pic tells us, "He looked like he was focused."

Of course, Colin was decked out in Nike gear -- no shock considering he has a deal with the Swoosh, which has stuck with him through everything.

Next up for Colin ... showtime on Saturday -- when roughly a dozen NFL teams will have reps at the Atlanta Falcons facility to evaluate the 32-year-old.

There's been a ton of stuff going down behind the scenes ... Kap's camp wanted the workout on a weekday so more NFL team decision makers could attend. He also wants a list of team reps attending the workout, something the NFL says it will NOT provide.