The discovery of an additional moon behind our current moon would depend on several factors, including the moon's size, distance from Earth, and its reflectivity or brightness. Assuming this second moon is relatively small and doesn't reflect much light, making it difficult to detect, it might take some time before its presence is noticed. However, with advancements in astronomical technology and increased surveillance of near-Earth space, it's challenging to provide an exact timeline.
That said, if we assume that the second moon is located relatively close to Earth, it might be discovered within a few years or even sooner. Numerous space missions, telescopes, and observatories continuously monitor the sky, scanning for celestial objects. The more comprehensive our observations and technological capabilities become, the higher the chances of detecting an additional moon.
It's worth noting that the Moon itself is relatively large and bright, making it highly unlikely for another moon to go completely unnoticed for an extended period. Nonetheless, the specific time frame for the discovery of a hypothetical hidden moon cannot be accurately determined, as it depends on multiple unpredictable factors.