I believe you are referring to a part of The Hound of the Baskervilles where Dr. Watson has been sent to keep an eye on Baskerville Hall, with regular reports sent by letter to his partner Mr. Holmes. Watson has already grown suspicious of Barrymore for two reasons: First, the fact that while Mr. Barrymore and his wife would not inherit any substantial money from the death of Mr. Henry Baskerville, they would inherit a little and serve as de facto owners of Baskerville Hall. The couple are already maid and butler of the house, rather high-ranking in the makeup of household staff at this time in England. With the house and its income at their disposal, they could live quite the cozy life! What's more, Mr. Barrymore has a beard, and Dr. Watson and Mr. Holmes were followed by a man with a beard in London.

During his stay at Baskerville Hall, Dr. Watson has heard a woman crying in the middle of the night—Mrs. Barrymore. Could the death of her employer have been so upsetting to her? Dr. Watson also hears footsteps, and decides to investigate them. Upon hearing the footsteps pass his door, he creeps out and spies Mr. Barrymore walking quietly down the hall with a candle. He enters an empty room, where Watson sees him place the candle in the windowsill, wait expectantly for a few minutes, and then leave. The next day, Watson explores the same room and discovers it has an excellent view of and from the moor. He comes to believe that Mr. Barrymore is signalling someone, and indeed he is. It is later revealed that Mrs. Barrymore's brother is an escaped convict who is taking refuge in the moors, and every other night, Mr. Barrymore signals for them to meet so he can offer him some food.