Ryu's cultivation wasn't deep enough to be qualified to sense Leopold's senses lock onto him, however, his Heavenly Pupils had no such limitations. The moment he felt a painful throb in his eye, he was immediately aware that his position had been locked onto.
The determination on Ryu's face didn't diminish in the slightest. Because the lock Leopold had on him was fading in and out, Ryu understood that this meant he was out of Leopold's solidified Spiritual Sense range. This meant that Leopold was using a technique to temporarily boost his senses, a technique that, in all likelihood, he couldn't use continuously. In this case, Ryu still had a chance.
After hours of running, Ryu had put several hundred miles between him and Leopold. Considering his estimate of Leopold's cultivation as at most a Connecting Heaven Realm expert, this meant that Leopold was at most in the Spiritual Endowment Realm, a step above Ryu's Spiritual Entrance Realm Mental Cultivation.