Nah. The same tech they use to scan for aliens would pick them up instantly. Unless they are dead.
To be an advanced civilization you must
- Send out radio waves
- Use some sort of propulsion(infrared waves/heat)
- If you understand chemistry, there is a high likelihood of you sending out both gamma and x-rays through fision and research
- Additionally, such a civilization would make a lot of noise. Sound travels better under water for receiving equipment
- You would notice such a group of people expanding as they slowly consumed resources in one area, like if...
- A severe shortage of fish in an area
- Lack of oxygen in one portion of the sea
- Waste would have to be expelled generating more thermal energy
- Have inorganic shapes and geometry in their advanced environment. This is how we find underwater cities
We already scan for this stuff constantly, so there is no hiding of people(or aliens) on this planet or solar system. Unless they are dead.