This may be far out in left field...but, Yorkies were bred to be ratters...do you think she is alerting you to a rodent maybe somewhere in a wall or in your attic?
As much as I hate to admit this...our neighborhood actually went through an infestation of attic rats after new construction in the hills beyond our track.
My Yorkie girls would stand at attention next to a wall and sniff & pace trying to find a way to get to their prize...ack!
Needless to say after some very nice professionals came to obliterate our new house guests...so did the sniffing & pacing!
