He cleared a dungeon that a party full of S and A ranks couldn't, which of course made them suspicious of him. Here's the thing: He hid everything from them, which made him look even more suspicious, but none of the things he hid were things he needed to hide, at least not from the one asking him. He could have told them the boss was a ghost, he had an ability to communicate with ghosts, he convinced the ghost to give up its obsession... He could have even gone into detail about his discussion with the ghost and the reward he got from the dungeon. HAD HE DONE SO, he would have made connections with a powerful guild. He could have established a resume that would get him brought to dungeons that he could easily help clear (because of that ability) to earn money.

I wrote too much but tl;dr he just hid his powers for no reason and it made him just look like an idiot.