Most trace gas detection methods developed so far largely rely on active sampling procedures, which are known to introduce different kinds of artifacts. Here, we demonstrate sampling-free in situ trace gas detection in millimeter scale volumes with fiber coupled cantilever enhanced photoacoustic spectroscopy. Our 2.4 mm diameter fiber-tip sensor is free from the wavelength modulation induced background signal (a phenomenon that is often overlooked in photoacoustic spectroscopy) and reaches a normalized noise equivalent absorption coefficient of 1.3 × 10 -9 W cm -1 Hz -1/2 for acetylene detection. To validate its in situ gas detection capability, we inserted the sensor into a mini fermenter for headspace monitoring of CO 2 production during yeast fermentation. Our results show that the sensor can easily follow the different stages of the CO 2 production of the fermentation process in great detail.