Why can we be so confident? I assume the probes would use the most minimal amount of mass possible (and still propel themselves and replicate). Our instruments

Why can we be so confident? I assume the probes would use the most minimal amount of mass possible (and still propel themselves and replicate). Our instruments are capable of tracking fair-sized asteroids in our solar system, but not tiny objects (which could have been designed to be difficult to detect). Though it's possible that the extent of their 'probing' would just be to announce their arrival in the widest swath of EMF possible.