[@Belwicket] she probably wouldn't be able to sense him, per say. I'm sure he's gone through a lot of trouble to keep that from happening. But Regina may be able to sense that something is wrong, that is, if her judgement hasn't been clouded by Isabeau. At first, she sensed an additional mind, and she sensed that someone was trying to get in. She couldn't tell who or what it was but had her suspicions, obviously, she kept calling the intruder a 'her'. Now she knows for sure that Isabeau is somewhere near. But where?! One thing she will be able to sense is Genevieve- the anger, the fear, the rage and possessiveness over her stone, and perhaps even the readiness to fight. Genevieve won't let Regina enter her mind, she'll be focused on her task at hand, but she is quite the predictable character and will be letting off all sorts of other things unintentionally for others to pick up on. And anyone keen to sensing energy could certainly sense a disruption, or some bad juju coming from the other side of the house. 

For anyone that saw Disney's Beauty and the Beast, think of this household like a smaller version of Beast's Castle. And instead of the East Wing and West Wing, there are the north towers and the guest rooms on the south wing of the house. And because it is a much smaller version, any battle that takes place in front of Genevieve's quarters, will certainly be heard through out the house. 

Edit - my post is up. I am open to collabs if need be