Only saw this one once in the movies, but here it goes. During the training scene where she is fighting the robots, towards the end, she does a slide under a robot with her guns in hand then releases them to do a back flip onto the robots back. Then the guns are back in the holsters instead of sliding across the floor.
You wrote the action sequence wrong... After Lara slides underneath the robot she begins to shoot it, it's later that she does a flip onto the robot's back, (she had never thrown her guns away!), after she begins to hit the robot with the guns and pulls out it's wires, she does throw her guns away, but when she gets off of it's back, you'll notice that she does pick them up again!