arXiv:2610.33367v1 Announce Type: new Abstract: This paper investigates physical layer security in waveguide-based pinching antenna systems with a mobile eavesdropper. Assuming perfect knowledge of the legitimate user's location and only statistical information about the eavesdropper's position, the ergodic secrecy rate is analyzed under Gaussian and practical finite-alphabet signaling. Since the exact ergodic secrecy rate does not admit a tractable closed-form expression, a closed-form analytical upper bound is derived using Jensen's inequality. The resulting expression serves as an efficient objective for antenna position optimization without requiring exhaustive Monte Carlo simulations. Numerical results verify the tightness of the proposed upper bound and show that the optimized antenna position closely matches the optimum obtained from Monte Carlo optimization. Furthermore, practical finite-alphabet signaling is shown to exhibit secrecy-rate saturation at high SNR, identifying an effective operating SNR region. Although the antenna position is optimized using the Gaussian-based upper bound, it remains effective for practical signaling.