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A few good answers have been posted here already, but I'd like to chime in with some more science-oriented comments.
To me, you can somewhat divide the possibilities based upon whether your society is ok with living an essentially subterranean existence, or if it requires at least some time on the planet's surface. But in all cases, heat, in one way or another, is a big problem.
Above-ground
The biggest hurdle to avoiding detection is heat. This is less complicated if your society subsists entirely on foraged foods and sashimi and wears nothing but leaves and animal skins. But since you say modern technology is available to them, I'm going to presume they indeed want to make use of that knowledge. That will require energy, and generating energy means generating heat.
For the foreseeable future, technologically modern societies require electricity to function. While living in a remote area is a good start for several reasons, even something as small as a campfire isn't guaranteed to escape detection out in the open. Even if you ensure all your heat sources reside indoors to some extent, that heat needs to go somewhere.
Let's consider some possibilities for generating power:
Solar panels are probably not your best bet. Although the technology continues to improve, solar power requires a large amount of space for a relatively small gain. You might be able to come up with some clever ways of camouflaging them from a distance, but in the end, massive arrays of warm, reflective surfaces would be extremely difficult to hide. Maintenance and disposal of broken-down panels is also an issue.
Hydroelectric is much more appealing, in my opinion. Instead of having a massive dam and plant facility, you could construct a system along a waterway that blends completely naturally into the landscape.
Relatedly, recent technology is getting better about not letting byproduct heat go to waste, and one notable usage is in water purification. This isn't limited to hydroelectric systems (e.g., this), but it obviously meshes well with hydroelectric infrastructure.
Nuclear power is also a good candidate. First, it's completely clean. If you do have to replace some spent fuel, it wouldn't be insurmountably difficult to dispose of a lump of radioactive metal in a covert way. Second, nuclear power plants can (and do) use water for coolant. In simplistic terms, reactor coolant is piped around in a loop; it absorbs heat from the nuclear reaction and transfers it away, cooling off as it travels around the pipe before getting back to the reaction, over and over. 435
It's certainly conceivable to construct a large underground network of piping to distribute the heat absorbed by the coolant water in a sneaky way. Such a facility would likely be more difficult engineering feat, however.
Another issue is, of course, food sources. You can certainly grow a lot of food in a small area, even with a complete lack of natural light. This, too, requires a lot of power and generates a lot of heat. But if you're able to hide your power generation facilities and mask the waste heat they put out, you can definitely do it with your food production methods.
Finally, there's the basic matter of "hiding" in itself. A good environment for staying hidden and one that lends itself well to our concerns is a jungle/rainforest biome. Jungles are excellent at hiding things. There are large swaths of unsettled rainforest/jungle across the planet with a) thick, natural cover from foliage; and b) watercourses traveling through rocky, uneven terrain (including caves, rapids, waterfalls, etc.). It's the perfect environment for disguising a technologically advanced society and the trappings it needs to maintain their standard of living.
Underground
A society living deep underground could be very difficult to detect. Again, thermal and electromagnetic energy can be perceptible from great distances and hard to hide. In theory, if most of your infrastructure is underground, it would be easier to mask your presence with the additional cover the ground above you provides, and the basic lack of physical evidence of your existence out in plain view.
But for the most part, a society living mainly underground would in fact have all the same concerns as laid out above, with some additional considerations.
First, the obvious point: humans weren't made to live in subterranean conditions. It's simply an inhospitable environment for us.
Here, again, there's issues related to heat. While temperatures below ground increase the deeper you go, this won't too be much of an issue if you don't plan on living miles and miles below the surface. Suppose your society plans to live in buildings that are still fairly deep -- let's say, a quarter mile/0.4 km below ground. In that case, the temperature would be about 20 °F/11 °C warmer than on the surface. That's a notable increase, but not insuperable. 258