Any civilization not worth contacting is worth eliminating: resource consumption plus a breakthrough could happen at any time changing the balance. Strike while they are stupid, blind and powerless.
If a civilization is intelligent enough to create camouflage masking its existence, you would not find it before it found you. Just the act of searching would reveal your existence which is why navy ships use passive radar and sonar.
Note I said “without hostile intentions”.
I understand it is hard to imagine alien civilizations that have no interest in eliminating others, but they are “alien”, and if they have serious high tech they could (among other things) simulate the natives so they could infiltrate at will without giving away their presence.
Terence McKenna has some fascinating “scenarios” that he discusses.
The best disguise would be as a non-intelligent species that could monitor activity and report periodically (simulating natural phenomenon for communication).
Sun Tzu (The Art of War) is full of ideas on how to carry out a long term strategy (that could really be used for peaceful or warlike purposes).
One final thought—are we really worried about dolphins or apes developing into dangerous enemies? If so, why would highly advanced aliens have such fears about us?