Why did Watson suspect Barrymore of being the person in the cab in London? Chapter 7

Sir Henry and Watson question Barrymore to learn that there are only two women in the house; he is certain that Mrs. Barrymore was not the screamer. However, when Watson meets Mrs. Barrymore, he notices signs that she had been crying the night before, and assumes that Mr. Barrymore has lied to them.

Watson therefore decides to investigate whether Barrymore was actually at Baskerville, as his telegram had indicated. He visits the postmaster at nearby Grimpen, who had insisted that he placed the initial telegram directly into Barrymore's hands. Upon interrogation, however, he admits that he actually delivered the telegram to Mrs. Barrymore, who promised to pass it along to her husband. Assuming Barrymore was the bearded man, Watson's only theory is that Barrymore was attempting to scare Sir Henry away from London so that he and Mrs. Barrymore could have the manor to themselves, but he admits that theory is inadequate.