Since Mars is no longer pristine (having been visited by numerous probes) couldn’t any bacteria have hitched a ride on one of the probes? That’s what scientists twenty years ago said about a Martian meteorite found in Antarctica. Remember? Slick Willie even said that they had found evidence of fossilized life in the meteorite from Mars. Now, he didn’t graduate from East Anglia University—but, his word is as good as the Gospel!
Yes that’s entirely possible. Contamination via the probes we sent there may be one source. Another, is analogous: it’s possible that some meteors from earth landed on Mars, carrying with them bacteria from here.
These will remain viable alternative theories for the reason there is bacterial life on Mars (if it is ever found) until and unless the DNA from such life could be shown to be so exotic that there would be no way it originated here.
In other words the discovery of bacterial life on Mars will not be that groundbreaking in of itself. It will have to be demonstrated that such bacteria came to be and evolved separately and independently in the Martian biosphere. If that could be shown, that would be monumental.
Otherwise we could just be getting excited about some distant cousins of terrestrial bacterial species already known.