Okay so people on IRC are all 'wut??' with that FEF3 'Secret' being the #5 overall, so let me explain:
FEF Secret #1: Hidden shop during first visit in Prixie's upper floors.If anyone would get to that one tile in Chapter 5 of FEF1, they would be transported to a secret shop run by Anja's cousin. She would sell -reaver and Killer weapons for a discount.
FEF Secret #2: Surprise treasure map in the tomb.This secret has been talked about for a while now; extra map filled with treasures after Chapter 8. Character backstory drama denied you it though.
FEF Secret #3: Choppy fun for Sheila aka 'Did No One Really Think Why I Sold a Boarding Axe So Early in FEF2'?If anyone stopped by this suspicious tile, they would find an axe embedded in a tree stump. It was a 'named' (don't remember it, though) Boarding Axe with a RES bonus. Missing on it triggered the ability to purchase normal Boarding Axe before Chapter 2x (the one with gargoyles at a harbor); if it would be found, the blacksmith wouldn't have a normal Boarding Axe in his stock.
FEF Secret #4: King Arthur Salvatore and the Lady of the Lake.During Epilogue Chapter C of FEF1 - If Salvatore would stop on any of 'deep' water tiles of the lake (he stopped at it's edge, twice - you were so close guys!), a beautiful water maiden would hand him a magical spear from the depths and bless him with S rank in Spears.
I cannot tell the weapon's stats because his imperial descendant in FEF2 will wield it - so you gonna find out anyway.
FEF Secret #5: Ancient Wyvern Rider King's Grave.The very first one you have found
It required Beryn to stop by any of the 'deep mountain' tiles. I thought that the 'strange' mountain tile at 18.17 would put you off, but alas, I can only bemoan your lack of instinct or secret finding skills
18.15 was the closest of the bunch and thus Beryn found it by chance. Gratz anyways!