Apologies for expanding on my own post here but could a potential alien civilisation actually comprise two (or more) cultures? Those travelling at near lightspeed and those that are planet bound, either the homeworld or subsequent colonies?
The travellers would spend years or decades travelling and maybe radioing back information, but time on the homeworlds would be at a different rate so if they did return it may be to an advanced technology or maybe even to the ruins of their world. Most of the traveller's subjective time may be spent in acceleration or deceleration and maybe probes sent from these ships at the start of deceleration may tell them if it is worth slowing more or just re accelerating.
Given this scenario would they bother to make contact with us? They may come across other travellers from other civilisations or evidence that they have visited somewhere or even other homeworlds; but given the vastness of space need they take the risk of contact? Focussed communication may take place but could we pick that up?
As for Earth, why take the risk? The probe shows an aggressive bunch with capability to send projectiles into their solar system and fit them with things that go bang. The traveller's ship could probably fry the lot of them but, who knows, a lucky missile could cause a lot of damage. Why bother? Re accelerate and keep going. Send a message out to the homeworlds with all the data you have from your own probes and the transmissions from Earth and leave it for another few centuries.
In other words we are not dealing with an advanced homeworld but a self sufficient expedition pretty well cut off from its homeworld so fairly risk averse. Our presence makes exploring the system not worth the risk. Water, fuel, minerals could be picked up or mined far out in theKuiper belt or Oort cloud without troubling us.
Crashed UFO's? Drone style probes that tell us nothing other than that aliens with an advanced technology exist. Oumuamua? Saw us and decided to move on!