How to find a plastered hole?
The slightest imperfections can be found on a wall with a bright light, usually brighter than a flashlight, although flashlights have been used before. It may work for you since you may have an idea where the spot is to begin with.

The trick is to shine the light across the wall, hold the flashlight or light against the wall so the light passes over the wall with as low of an angle as possible. The flashlight or light is almost parallel as possible. This way the texture of the wall is more prominent, because every high spot, even the tiniest, casts a shadow. Since the repair has been made recently and has less coats of paint over it, the texture over the repair will be slightly different than the wall surrounding the repair that has more paint on it.