Yes, I would think so. But look at the habitats they occupy. Generally places where we would not try to build anything (which is how many prehistoric dwelling or grave sites are discovered, during excavation for construction). Builders of roads and railroads seek out the routes needing the least modification when going through mountainous regions, and you can hide just about anything in a forested swamp.
Whatever they are (unless they are in fact a hoax perpetrated over hundreds of years by multiple cultures), God made them; if He wants us to know about them we will make the required discoveries.