With a Li-Fi network, any intruder needs to be in the range of two Li-Fi routers where network activity is going on. Li-Fi signals will not penetrate walls or windows, unless the signals are directed through a window on purpose. Also, in this case the signals will not be detectable outside the range of the transmitter.
Since data signals are transmitted via laser, they do not spread outside the laser beam. Also the strength of the signal is so precisely adjusted to the receiver that there are no reflections readable.
Intruders will not be able to access the Li-Fi network, without being physically positioned within the line of sight between transmitter and receiver of the active devices.