Here are my assumptions. If they are correct, then my conclusion must be correct. They seem plausible at this time, but are far from certain.

1. Live emerges wherever conditions are conducive for it for a sufficient time. To be explicit, my requirements for conducive conditions are liquid water, combined or free nitrogen, combined cargon, combined oxygen, trace minerals (iron, magnesium, chromium, etc), temperatures from 0 to 40 C, pressures from .01 to 10 bar. A sufficient time is at least 100,000,000 years.

2. Conditions on Mars were conducive for the emergence of life during the early days of the planet's existence. There is evidence of liquid water, there are plenty of trace minerals, there are nitrogen and carbon dioxide in the atmosphere. These persisted for at least 100,000,000 years.

3. Life is extremely robust. Once begun, it rapidly evolves to meet changing conditions. For example: There is bacterial life in the rocks of the ice-free valleys of Antarctica. These conditions are even more extreme than the equatorial regions of Mars.

1 & 2 imply that life emerged on Mars in its early days.
If life emerged on Mars, 3 implies that life is still there.

This is my reasoning. Of course, my premises may be wrong, and therefore my conclusion also. However, these are reasonable premises that are accepted by at least a large minority of scientists. Therefore, I predict that microbial life will be found on Mars.