Volcanic activity
the main sign of this is heat. We haven't seen any hotspots on Mars
second sign is escaping vapour - mostly water - we haven't seen this on mars either
third sign is fresh lavas - we haven't seen THIS either...
methane can be produced by slow chemical breakdown of Ultrabasic igneous rocks, deep under the surface, producing the metamorphic rock we call serpentinite. The rocks that are being reacted may be millions or billions of years old.