The two Viking landers, 6500km apart, had positive results. That would suggest that life is widespread and close to the surface. But the atmosphere of Mars is in chemical equilibrium, which means life can't be widespread and close to the surface because it is not affecting the atmosphere. If life is present on Mars it would have to be deep underground, perhaps in a geothermally heated brine lake, which would be very hard to detect with current technology. The Viking data was probably non-biological, it really isn't strong evidence for life today.