Very recently, we have performed a couple of experiments, KEK PS-E549/E570, for the detailed study of the strange tribaryon S 0(3115) obtained in KEK PS-E471. These experiments were performed to accumulate much higher statistics with improved experimental apparatus especially for the better proton spectroscopy of the 4He(stopped K −, N) reaction. In contrast to the previous proton spectrum, no narrow (∼ 20 MeV) peak structure was found either in the inclusive 4He(stopped K −, p) or in the semi-inclusive 4He(stopped K −, pX ±) reaction channel, which is equivalent to the previous E471 event trigger condition. Detailed analysis of the present data and simulation shows that the peak, corresponding to S 0(3115), has been an experimental artifact. Present analysis does not exclude the possible existence of a much wider structure. To be sensitive to such structure and for better understanding of the non-mesonic K − absorption reaction channel, detailed analysis of the data is in progress.