Is there any rough figure on when the last time was that surface water existed?
After impacts from space, water vapor is produced from soil ices, which provides a temporary microclimate and atmosphere of water vapor, making it possible for liquid water to flow.
This accounts for that from-nowhere-to-nowhere erosion patterns.
With the increase in surveillance of the Martian surface (from orbit and from the surface) this will be observed at some point as it's happening.