Mu Feichi's intelligence department was not located on the main mountain of Tianyu Mountain. One needed to enter and exit from the secret entrance at Tianyu Mountain. Yun Xi and Mu Feichi traveled all the way from the bumpy road at the foot of the mountain to yet another mountain within the area.
Among the five big mountains that the President gave to Mu Feichi, the Mu Mansion was built on the top of Tianyu Mountain. Another mountain housed a training camp, while another one was the location of Mu Feichi's most hidden intelligence department.
As for what was on the remaining two mountains... Yun Xi didn't dare to ask. Each secure location was armed with military checkpoints and sentries. Surveillance cameras captured every move and transmitted them directly to the monitoring room.