Agreed, locally the aircraft is subsonic. However, air at lower levels will probably be moving at much les than the M0.15 in which our hypothetical aircraft is travelling.
So if the disturbance caused by the aircraft reaches the ground it will be propagated at a speed greater than the local speed of sound. So a shockwave should form somewhere between the aircraft and the ground. I suspect that at the distance where this is likely to happen the disturbance will be so attenuated that the phenomenon is not easily observable.
But can it occur and has it been observed?