Recent ground and space-based observations show that stars with multiple planets are common in the galaxy. Most of these observational methods are biased toward detecting large planets near to their host stars. Because of these observational biases, these systems can hide small, close-in planets or far-orbiting (big or small) companions. These planets can still exert dynamical influence on the known planets and have such influence exerted upon them in turn. In certain configurations, this influence can destabilize the system; in others, the star’s gravitational influence can shield the system from destabilization. For example, in systems with planets close to the host star, effects arising from general relativity can help to stabilize the configuration even if there are undetected planets orbiting close to the host star. We derive criteria for hidden planets orbiting both beyond and within known planets that quantify how strongly general relativistic effects can stabilize systems that would otherwise be unstable. As a proof-of-concept, we investigate the several systems and show where within them an undetected planet could lie.