Humans have achieved FTL and have colonized several planets in our greater stellar neighborhood. While we have discovered several planets that harbor life as we know it, we have not yet discovered other intelligent life. However, this isn't because we haven't yet encountered it; within the same stellar neighborhoods we've come to inhabit, there exist one or more intelligent and technologically advanced civilizations, none of which have yet become aware of each other or us. It may even be the case that a single star system plays host to all of them simultaneously, and they still haven't found each other.

How do you keep FTL-capable, intelligent, advanced civilizations from noticing or even bothering each other?

EDIT: Constraints: The creatures themselves must be similar enough to humans that they could be easily recognized as intelligent if encountered in-person. Individuals should be roughly similar in size to humans (they could be as large as whales or as small as bugs, but not, for example, microscopic or telescopic in scale.)