The problem with seeing a "smoking gun" on Mars is context.

Mars is not Earth.

If you see a meandering river bed on Earth then it is reasonable to conclude
that water flowed there recently. After all, river beds are made by rivers and rivers are flowing water. However there is a tiny assumption included in this
line of logic: River beds on earth are typically only a few millenia old. Rivers meander, so by definition river beds are geologically young. However almost all of the large surface features that we see on Mars are extremely ancient, e.g. hundreds of millions or even billions of years old. A hundred fifty million years ago, the Mississippi River would not have existed. During that same time the continent of South America and Africa had not yet separated. That's a key concept to keep in mind: Almost all of these "river beds" on Mars are more or less the same now as they were prior to the Earth's continents breaking up into smaller continents. Our Earth based intuition only leads us to faulty conclusions when applied towards Mars.

Gary