arXiv:2610.33142v1 Announce Type: new Abstract: Smartphone use while walking reduces environmental awareness and can lead to accidents and injury, yet existing obstacle-awareness systems remain unadopted because they require users to hold the phone upright and aimed forward, conflicting with how people actually hold their devices. We introduce GlanceAR, an always-on mobile obstacle-awareness system designed for natural phone-holding postures. GlanceAR utilizes a low-cost passive mirror attachment to redirect the phone's rear LiDAR and RGB camera forward without additional electronic components. In a within-subjects study with N=11 participants navigating indoor and outdoor obstacle courses while engaged in a primary on-phone task, we evaluated three sensing configurations: GlanceAR, a LiDAR-only variant for privacy, and an attachment-free ultra-wide camera baseline. Our findings reveal trade-offs among sensing fidelity, obstacle cue design, and social acceptability, and surface insights around AI complacency and the tension between on-screen task focus and physical safety.