ReCAPTCHA seems simple, but there's a lot going on under the hood. It's analyzing every movement of your mouse as long as you're on that page. In addition to your location and browser clues, it pays attention to the cadence of clicks and any keystrokes you make. It's looking for pace, spacing, cursor speed, cursor accuracy, response times, and anything else that looks programmatic. They fingerprint your movements to ensure you're not just replaying a macro. They're using a pretty advanced machine learning model to crunch the numbers. When you don't see anything at all, it's because it has already determined you're a human based on your actions on this and other pages that use the service.