For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm-1 located in ν3 band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor's usability in environmental monitoring.