not all guards end their shifts at the same time.

Note whether the shift changes are staggered, i.e. Learning the schedule of the guard shifts is also valuable. One exception is of course if there are a small number of guards during off business hours, it still may not be the best time as any movement will be seen as abnormal and will draw attention. Obviously, attempting the assessment when the fewest possible number of guards is on duty is generally best. Knowing the number of guards on duty on any given shift will also give you an idea as to plan your penetration assessment. Knowing where the guards are is invaluable when you wish to avoid them. By listening to radio traffic on the guard frequency, it is usually possible to learn the schedule of the guard rounds as well as any regular movements. The information in this book is especially valuable when the target organization has a guard force that uses two-way radios.

Chris Sanyk, in Wireless Reconnaissance in Penetration Testing, 2013 Who is Guarding the Guards?

Traffic monitor