Now-a-days, it is common to find multiple WLAN deployments coexisting in shared spaces. The resulting interference between transmissions represents an important source of performance degradation, especially those originating from hidden traffic. This note explores the relation between hidden traffic and packet losses, using renewal theory to show that losses constitute biased samples of hidden traffic. Building on the developed analytical model, the paper derives the optimal frame length that maximizes throughput in the presence of hidden traffic. The results are validated using an 802.11 WLAN in-lab setting.