If there are so many “signs” of aliens, how can we be sure that there are other beings living around us, and how can

Question: If there are so many "signs" of aliens, how can we be sure that there are other beings living around us, and how can we be sure that they would have got so sophisticated that they would have sent us a message by now?
- Keywords:
  alien,
  earth,
  message,
  space
Asked by williamtierney to Ben, Clare, Ezzy, Mario, Sam on 16 Mar 2012.
- Mario Campanelli answered on 15 Mar 2012:
  
  ?? where are all these many “signs” of aliens?? I have not seen any!
- Clare Burrage answered on 15 Mar 2012:
  
  I don’t think there have been any signs of aliens. We know that there are other planets in our universe that could have life on them, but we haven’t seen that life yet. Some people have claimed to see signs of alien life on Earth but they have all been proved to be wrong.
- Elizabeth Pearson answered on 16 Mar 2012:
  
  There’s never been any sure signs of alien life. We’ve found things that people thought were signals from aliens but turned out to be something else.

Question: If there are so many "signs" of aliens, how can we be sure that there are other beings living around us, and how can we be sure that they would have got so sophisticated that they would have sent us a message by now?