Teresa's arm went completely numb before she died, according to Irene. 

BOB's twisting hand is the first thing that emerges from the Glastonbury portal.

Mike cut his evil arm off.

I think all of these things are related, possibly to the LMFAP ("the arm") but who can say how for sure?   

I can't remember where I read it, but I heard it was directly related to BOB's emerging from the Lodge on his own (which could only happen when the stars were right, as Lovecraft might say). Because of the conjunction of Jupiter and Saturn, the Lodge entrance was somehow more open. Like the veil between worlds was thinner at that time. This allowed BOB to squeeze his hand and arm through, and then emerge completely at the tail end of an episode (when he said "I'm out." and you could see a closeup reflection of the red curtains in the oil pool on the ground).

BOB was still in some sort of ghostly form when he emerged from the Lodge, as he wasn't walking around inside of anybody when he did so. So BOB cannot typically be sensed without some psychic ability, etc. But the hand tremors multiple characters displayed at the end of Season Two was an indication that BOB was nearby; perhaps even in the room with the flesh and blood living whether they knew it or not. The tremors might have been a sign that BOB was trying to creep inside of various people, looking for new hosts to inhabit and act as instruments for the powers of the Black Lodge. Possession seems to start with parts of the body at first, such as the extremities, as the possessing spirit eases its way into the host. It doesn't just completely overcome the host in an instant, like flipping a light switch.