You can mentally say "thinking" to yourself as a further reminder of what just happened.
When you notice that you have gotten so caught up in thoughts that you have forgotten that you're sitting in the room, just gently bring yourself back to the breath. You can mentally say "thinking" to yourself as a further reminder of what just happened. This labeling is not a judgment; it is a neutral observation: "Thinking has just occurred." I like to think of it as a kind of weather report: "Thinking has just been observed in the vicinity."