What is behind Holmes' decision to return to London after sending Sir Henry to Merripit house? In Chapter 11

Holmes does not return to London. He pretended to be in London, so the suspects would not be on guard while Watson and Sir Henry investigated. He needed to gather information that would not have been easy to get had he made his presence known, thus, he allowed everyone to believe he'd been in London.

“My dear fellow, you have been invaluable to me in this as in many other cases, and I beg that you will forgive me if I have seemed to play a trick upon you. In truth, it was partly for your own sake that I did it, and it was my appreciation of the danger which you ran which led me to come down and examine the matter for myself. Had I been with Sir Henry and you it is confident that my point of view would have been the same as yours, and my presence would have warned our very formidable opponents to be on their guard. As it is, I have been able to get about as I could not possibly have done had I been living in the Hall, and I remain an unknown factor in the business, ready to throw in all my weight at a critical moment.”