The surface plasmon in nanoscale thin Ag films deposited on Ni(111) was studied as a function of the impinging electron beam energy and of the scattering angle. The nonmonotonous behavior of the amplitude of the Ag surface plasmon suggests the occurrence of interference effects between the incoming and the outgoing field. Such interference phenomena occur whenever the scattering plane is in the close vicinities of the jellium edge. An alternative explanation involves diffraction effects.