The First Model-Independent Upper Bound on Micro-lensing Signature of the Highest Mass Binary Black Hole Event GW231123

The recently discovered gravitational wave event, GW231123, is the most massive binary black hole merger detected to date. The inferred source masses of the event fall within the pair-instability supernova mass gap, where black holes formed directly from stellar progenitors are expected to be rare, making alternative formation scenarios for such massive black holes especially relevant. One proposed explanation is gravitational lensing, which can make the source masses to be inferred as higher than their true values. In this work, we search for lensing signatures in GW231123, together with other O4a events, using a model-independent approach with mu-GLANCE. The method tests residual strain for correlated features across the detector network via cross-correlation and infers lensing-induced modulations within a Bayesian framework. Our analysis finds no strong evidence for lensing in GW231123, but reveals a potential residual feature that could be consistent with microlensing, with a modulation amplitude of up to 0.8 at 95% confidence. However, we find that waveform systematics for such heavy binary systems are sufficiently large to shadow the lensing signatures in short-duration signals like GW231123, preventing any definitive claim of lensing at this stage. We conclude that, if this event is lensed, similar lensed events will be detectable in the near future with current detector sensitivity, opening a new discovery space for lensed gravitational waves with the aid of more accurate waveform models.