Small Shield Volcano with "Summit Caldera" (Saturated and Enhanced Natural Colors; credits: Dr M. Faccin - Lunexit Team)

Although there are a few truly giant Shield Volcanoes on Mars, there are also many smaller ones.
There's a strong interest in imaging the Volcanic Vent Regions of Mars, in order to understand not only the Volcanic Processes themselves, but also to search for any signs of recent activity.
In fact, it has been suggested that active volcanism is one possible explanation for the Methane gas that has been detected in the Atmosphere of Mars.

This HiRISE image shows that the Summit Caldera of a small Shield Volcano is mantled by Dust and covered by tiny Impact Craters. And it is just this last element (such as the existence of Impact Craters all over the Caldera) the one that allows us to believe that there are basically no chances that the imaged Volcano was active recently enough to affect the Atmosphere of Mars through the release of any gas whatsoever.