What does Poole and Utterson believe has happened to Dr. Jekyll and why

In Chapter Eight, Poole reveals that he has come to Utterson in desperation. He is severely concerned about Dr. Jekyll's well being, as the man has been locked in his cabinet for weeks and allows no one to see him. Poole admits that he believes there has been "foul play", but refuses to go into specifics. Utterson has long been suspicious of Jekyll's behavior and has worried for his friend.

"O, sir," cried Poole, "do you think I do not know my master after twenty years? Do you think I do not know where his head comes to in the cabinet door, where I saw him every morning of my life? No, sir, that thing in the mask was never Dr. Jekyll—God knows what it was, but it was never Dr. Jekyll; and it is the belief of my heart that there was murder done."

"Poole," replied the lawyer, "if you say that, it will become my duty to make certain. Much as I desire to spare your master's feelings, much as I am puzzled by this note which seems to prove him to be still alive, I shall consider it my duty to break in that door."