what does "F-5" mean?
At the bottom, at Secret Service Post F-5, Agent Bob Foster of the kiddie detail joined them.

Question

Answer

A post is some place where people are stationed to perform some official task.  In this case, 'post F-5' refers to some official name for some place where people in the secret service would meet to be assigned work and gather information.
Because there is no context here, we don't know where that is.  It might refer to some office or other physical place.

Don’t miss out on the opportunity to learn a language from the comfort of your own home. Browse our selection of experienced language tutors and enroll in your first lesson now!