Does it have to be really that "high tech" to be "undetectable"? Today we're happy if we can detect asteroids of thousands of tons as they're floating towards u

Does it have to be really that "high tech" to be "undetectable"? Today we're happy if we can detect asteroids of thousands of tons as they're floating towards us. A probe meant for interstellar travel just doesn't have the energy to keep flashing all the time. I seriously doubt that we can detect a few tons of unspecified material, traveling at high speed in some distance from Earth, with big enough probability. Thus, the only statement we can make is "no probe designed to make contact was seen in our system in the last <100 years". This does not sound as such a huge paradox to me.