I'm not sure I understand this whole contamination argument. There are so many ways to find signs of life, it could be a fossil, it could be frozen in ice deep

I'm not sure I understand this whole contamination argument. There are so many ways to find signs of life, it could be a fossil, it could be frozen in ice deep underground it could be evidence of non-Earth compatible biology. Sure, once humans are there we should check every alien life discovery with unprecedented rigour, but there could be evidence which can't be explained with "humans brought it with them" type of reasoning.