Never done this in r/askhistorians, but can I ask a question that expands on this one a bit? I know a lot of previously "lost" civilazations were found or recognized due to their attestation elsewhere in the historic record. The Hittites, for example, were a people about whom next to nothing was known other than they kept bothering the Egyptians...until scholars unearthed bits of Hittite script in Anatolia.

Are there any other cultures or peoples whose existence is strongly hinted at by trade agreements, war monuments, or cross-cultural documents, but who for lack of a better word can't "speak for themselves" that historians and archeologists are out there trying to find more evidence of?

Exploration geology has some insight to this problem as part of the job is finding new mineral deposits in the same way archaeologists are looking for new places to dig. The critical information to answer your question would be where and how often were "lost civilizations" discovered previously. The rate of past discoveries might predict future discoveries. (Unfortunately, my suspicion is that the idea of a "lost civilization" may be a bit tough to reliably define.) Based on mineral exploration, the best place to look for a "lost civilization" might be where previous ones were discovered. The common quote is that "the best place to look for elephants is in elephant country". (A bit of speculation is required for a speculative question...)

I don't know how anyone could possibly offer an accurate answer to your question, but there are certainly lost civilizations yet to be found. David Grann's "Lost City of Z" describes the author's adventures in trying to track the lost explorer Percy Fawcett who disappeared in the Amazonian jungle looking for the Lost City of Z. Dozens of people had died trying to find Fawcett, but Grann, a New York city native decided to try anyway. In the end, he actually found a site that may very well have been the elusive city. It had disappeared because it was probably mostly made of wood, which makes sense. If the people built their structures from wood, rather than carved from stone, very little would remain hundreds or thousands of years later. Civilizations that recognized that stone structures would offer a lasting legacy are known, but how many civilizations used native materials for their structures - wood, tree trunks, reeds, mud, etc.? Cultures that had to put the bulk of their daily effort into hunting and gathering food wouldn't have had the luxury of time to find, haul, carve, and assemble stone structures, they were too busy trying to survive from day to day. A structure that could keep the rain off and the animals out was good enough. Those sorts of cultures would have melted into the landscape over the centuries, leaving little to show for their presence.

On a personal level, I used to know a guy who would go on solo backpacking tours along the Mexican coast, stopping in little villages and asking the locals if they knew of any ruins in the nearby jungle. He would often find someone who would take him into the jungle and show him completely untouched ruins, which he thought were probably Mayan. As far as he could tell, these sites were completely unknown to archeologists and showed no signs of ever being explored by anyone. He knew of many sites like this. While we know of the Mayans, we don't know about these small lost cities all along the Mexican coast.

Besides being dangerous, I thought his personal explorations were amazing, and told him he should contact somebody about them, but he wasn't interested in taking it any further. He had seen them, and that was good enough for him.

A few years ago I worked on a project that was looking to use GIS/geomorphology to reverse-engineer where we should be looking for lost cities. It's not an entirely new approach (look for village remains where there is fresh water, food, etc.), but the technology was getting better for it at that time.

We found a late Bronze age settlement with dozens of buildings, and probably many more that had sunk into the sea as the shoreline there had gradually descended over time. We certainly found a ton of artifacts in the water that would imply that there was a lot more of a city we couldn't fully investigate.

If you are curious about it, it later turned into this project: http://mailer.fsu.edu/~dpullen/SHARP/index.html

The website doesn't do it justice, but basically the day we "found" this place, we had a couple of divers out looking at the shelf off the coast, and a few of us were walking along the shoreline looking for artifacts (in a lot of Greece, there is so much historical detritus that you can just walk around and find pot sherds, stone tools, etc on the ground). One of the geologists was walking the shoreline with me and we weren't finding all that much, so she decided to walk up into the hills just inland while I continued on the shore. A few minutes later she called down to me to come up and meet her. She found a stone wall. When we started walking around, it only took a few minutes to realize we were in the middle of a huge number of buildings. What was left of the walls were only 2-4 feet tall in most places, and the whole area was overgrown in thick brush and brambles, but once you got down in it, it was just amazing. We were gobsmacked.

Spent a few days there mapping some of the buildings out and trying to understand what we'd found. I later found a cache of literally hundreds of obsidian blades along the shoreline. Not sure what else it would have been except the work of a craftsman who was just churning the things out.

A few of the local goat herders knew there were walls down there. My impression was they didn't really think or care too much about them.

Here's my best guess about lost civilizations: there are probably not all that many places that no one on earth knows about (though things covered by jungle/desert/sunk into the sea are always possible). There are probably plenty of places people don't know the significance of, or places that people haven't publicized because they don't want their land seized or what have you. That was always a concern in Greece. Farmers and shepherds didn't want to tell people about the walls they stumbled onto or the objects that came up when they plowed their field because they were afraid the government might come and take their land away from them. I'd be surprised if other places in the world are much different.

Any answer you are given will likely be wrong. No one really knows. Much of history has never been recorded, some will never be decoded if it was recorded, some has been translated wrong and much was destroyed by subsequent civilizations. We can just be grateful for what ever we find and understand.