The origin of methane on Mars is still unknown. Methanogens may be the source & if so would prove life exists on Mars, but no-one yet knows. As to the depth of the source of methane on Mars, this too is unknown.
The presence of methane in Mars' atmosphere could be of geological origin.
One way to confirm the biological origin of methane would be to measure the isotope ratios of carbon and hydrogen, the two elements in methane. Life on Earth tends to use lighter isotopes, for example, more Carbon-12 than Carbon-13, because this requires less energy for bonding.
What is known is that the levels of methane in the atmosphere of Mars varies, but the variation is not due to seasonal changes. The reason for the variation is still unknown.