If a player indicates that they believe an NPC may be not telling the truth, generally an insight check is called for. If I know the NPC is being honest, I still call for a roll. If the player suspects they are being followed but they are not, I'll still call for a perception check.
In either case, I don't want the player to be given information the PC cannot have. They will be fairly certain of the facts with a high result, uncertain with a low. If I just said no to the insight check the player knows, guaranteed, that the NPC is truthful. In the latter case, telling them not to bother with a perception check guarantees they aren't being followed.
There is technically no uncertainty as far as the state of the world, I'm maintaining uncertainty in the knowledge of the world from the PC's perspective.
I don't consider that fudging a roll. I always roll in the open except on contested checks. Those I may, in these cases, roll secretly and ignore the result.