There is something to that - a really thorough search will, however, ultimately require boots on the ground, in and of themselves altering the environment. Some kind of an interplanetary Heisenberg rule…
While I think it will require a really huge colony to start having any noticeable impact on the environment of the planet as such, we should face the fact that once a human sneezes on another celestial body, it will become irreversibly infected with terrestrial microflora, irrespective of how many air vents may be in the way. And some of that microflora will probably be able to survive in the harsh environment. And a small subset of that microflora might be able to thrive.
So from one point of view - a catastrophic ecological contamination. However from the other point of view - we could be the Martian panspermia. Planting life on another planet which may give rise to completely new organisms, millions of years after the last colonist die in their glass domes.
