I'm not sure I fully understand the question, but TIRF limits excitation to a thin plane directly above the coverslip. The exact depth depends on the wavelength and angle of incidence, but is typically in the range of 100-200 um. So cell surface proteins that are visible by TIRF are within this range of the surface. I would think mechanical changes of adhesion complexes and internalization allow some proteins to dissociate or turnover without sticking to the surface, but to some extent surface proteins do get stuck to the glass. Close examination of the path of migrating cells will reveal a trail of membrane and protein left behind.