How do I handle when my players overestimate the importance of an encounter/a location?

TL;DR: How can I (and should I) make the players’ efforts worth their while when they overestimate the importance of an encounter or a location?

I am running a homebrew DnD 5e adventure where the players’ goal is to find the grove where the witch has kidnapped some children. Nearby there is a mansion where the local lord lives. The initial plan was for the players to be able to go to the mansion and find some clues to the witch’s location.

However, as the players arrived, they became certain that the children are actually inside the mansion, and started devising elaborate plans to sneak inside. Having spent almost a session on sneaking through what I thought would be a fifteen minute "walk in through the open gate, ask some questions, continue", I feel like I now have to provide some more reward for the players’ work than "the princess is in another castle".

What are some strategies to tackle players spending too much time on things that were meant to be minor?

I see three options:

  • Making up some reward in terms of resources that can be useful in the climactic finale
  • Giving no reward more than the initially planned small clues, as the players were simply unlucky with the path they chose/failed to solve the puzzle I presented
  • Being clearer that this encounter is not going to yield a satisfying reward (but this time around it’s too late for that)

On a meta level, I am wondering about how to deal with such situations in advance. Is it a good idea to plan each encounter/location with different rewards based on how much time players choose to spend there? Or rather to redirect them away from unimportant locations through narration?