A few thoughts:
1. I've had that experience before. I made a note of it as to which specific owner(s) I suspected were FBG and made sure to take that into account as to whether or not to draft a player ahead of his FBG recommended slot in future drafts, or even the current draft once the trend surfaces.
2. As I'm sure you know, ADP is just that, average, so half the drafts will usually see a player go earlier than ADP, if only by a pick or two. And of course, recent ADP can be different from summer-long ADP.
3. I've used a second source and weighted my positional ranks to help differentiate and/or supplement my tiers. Getting a second viewpoint forces me to weigh on which ranking I trust more at various positions. This year, I went 60 FBG/40 brand X at RB, 55 FBG/45 brand X at WR, 45 FBG/55 brand X at QB, and 75 FBG/25 brand X at TE. Best of all, I'm forced to consciously examine exactly how I feel about nearly every player I will consider drafting by going through that exercise. If you use a GOOD second source, then you may tilt the players away from the straight-FBG ticket and move some players up or down enough to avoid missing on all or most of them.
4. Speaking of missing on players, I've never had a draft where I got all the realistic players to fall to me that I wanted. This year, I was sandwiched between two very good drafters, and I was more often than not seeing the top player on my list go immediately before my slot. Hopefully, I returned the favor often. But I always use tiers to help me see where the value drops are and nearly every time, I was able to get a comparable player. That tier strategy may become increasingly important if your league becomes heavily populated by FBGs.
And it's even possible that one of your FBG suspects was coming to similar conclusions to FBG either by their own judgment or with the advice of another FF source. A few of the players I barely missed were Ridley and McGahee at RB, and Stevie Johnson, Lloyd and Torrey Smith at WR. The Lloyd and Smith drafts were by someone who I know is not FBG. They may be the worst team in the league post-draft. Sometimes there are blind stabs in the dark. Some owners like to load up on high-scoring teams like the Patriots (as in 2 of the 5 names I just listed.). And it's not like the Broncos have been a secret this year with all the Peyton coverage, thus explaining McGahee as a common mid-round RB championed at many places, not just FBG.
Last year in a long-running stable league I was annoyed because my plans to get either Graham or Gronk in the sixth round were foiled by two owners who took both of them just a handful of picks before my plan could be enacted. Imagine how frustrating it was seeing them both go off while I fooled around with Owen Daniels and Fred Davis, especially when I played against the Graham owner in the championship while Davis was out due to his suspension.
I didn't realize it at the time, but when I looked at both teams, I saw that each had drafted to match the TE with their QB. Apparently that was the big strategy, the so-called "connection" and, being that neither is FBG nor necessarily shark-like, they were filling out their starters rather than going best RB/WR available. This story shows that sometimes, there's just very little you can do and the picks don't fall your way. All you can do is prepare sufficiently to dig deep as needed at certain positions when you get sniped.
