I don't know about the one in Finland, but the one in New Mexico is in the Permian Basin[1]. There are oil and gas resources all over the place. There are also Potash mines right next to the New Mexico facility (and it itself is a salt mine). Leaving it unmarked poses a real risk that someone someone will drill or dig into it accidentally looking for resources.

[1] https://en.wikipedia.org/wiki/Permian_Basin_(North_America)