We wouldn't be able to see the probe itself, rather we'd see its laser. The planet outputs (or reflects) vastly more total light than the laser, but the laser is pointed directly at the receiver and can output more light in that specific direction than the planet does. Plus, to get interesting data about the planet we need to get quite a lot of light from it, whereas we can get data from the laser over time by pulsing it so long as we can get any light from it at all.