Well, I found it and I first want to thank Action Dave and others who suggested that I use a GFCI breaker in place of a regular breaker. There was a dead short between the neutral and the EGC. I had to split apart all the wiring (blacks, whites and grounds) in (3) 3-gang switch JB's and a receptacle JB in order to trace the problem. On the affected circuit was the lighting in the 1st Fl bath, the hallway lighting, outdoor lighting, receptacles and lighting in the office. It ended up being in a 3-wire/3-way switch NM cable for the hallway lights. I wasn't able to isolate the problem at the recessed lights so I'm guessing it was a staple in the wire. I ended up cutting the EGC short at each end and installing a small wire nut on that EGC. I figured that was the best way to remedy this rather than cut the walls and ceilings open. This really threw me for a loop because the only time the AFCI tripped was when the bath fan was turned on or when the office lights were turned on. It never tripped when the hallway lights were turned on.
As luck would have it, an EC friend of mine called me while I was on this service call with another AFCI problem. He remodeled a portion of a kitchen and had to install an AFCI breaker for one of the circuits. He had installed that circuit in a 60A sub-panel. A carpenter was working in another area of the house where he plugged his saw into a receptacle fed off the main breaker panel. Each time he used his saw it tripped the AFCI breaker in the sub-panel. Any thoughts on this ? I don't care how weird you think they might be. I can believe anything at this point.