Found: Retired/retiring spy or PI finds a hidden tunnel and encounters a ghost

I read this book in the early 90s, and I remember thinking that it was probably from the previous decade or before.

All I remember from the plot was that a private investigator or spy was going on a vacation (or perhaps retiring) and was staying at an inn with all of his spy/PI gear (he had an "attaché case with false bottom", if memory serves). I remember that last part specifically because I recall a part of the book where the protagonist is in a dark, dripping tunnel he found that leads to an abandoned castle or mansion. In that tunnel he stops to reconsider what he is doing (I think his flash light goes out), and he mentions that no one was on the other end of his hidden radio and that he was alone. In the end, he finds himself in the castle or mansion and I *believe* he meets a ghost or spirit of some sort and befriends it.

It was a relatively short children's book with full page illustrations on every page. The illustrations themselves were pen and ink with a lot of mustard yellows.

I would love to find this book and read it my son, who is turning 1 in a few weeks. I would appreciate any help this community can provide! Thank you!

I've been searching for this book off and on for years now and I finally found it! I remembered that the part in the tunnel was under a lake; I added that to my ever-changing Google search and there it was! The Haunted Spy, by Barbara Byfield!! Hopefully this helps someone out.