Membrane potential fluctuations have previously been detected using second harmonic (SH) water imaging on neuronal cells and model lipid bilayer membranes. We report that such fluctuations are also visible when membrane potential-sensitive fluorophores are used as contrast agents, and fluctuations are imaged on both free-standing lipid membranes (FLMs) and on the plasma membranes of neuroblastoma cells. We show that upon K+ depolarization, non-uniform recovery responses occur across cells and within single cells. We discuss the origins and implications of such fluctuations, and investigate the molecular-level details of membrane potential distributions on FLMs and compare it to those on giant unilamellar vesicles (GUVs). SH water imaging shows that the hydrated part of lipid membranes is most likely composed of regions having a diffuse double layer, and other regions having an additional condensed double layer, with a high concentration of ions/ionic groups. In terms of transmembrane potential distributions, FLMs and GUVs show similar signatures, as expected from electrostatics. Comparing passive ion transport, FLMs and GUVs of identical composition behave differently, with GUVs being more permeable for proton transport (similar to 20x). This is likely caused by differences in the hydrophobic cores of the membranes, which create different energetic barriers for the proton transport.