Go to bed....

Me:  Heidi Go to Bed!

Heidi: But Mom, you don't know where I am?  Where am I? 
Me: You're in your room by the heater vent (I was in the kitchen washing some dishes)
talking into the vent. Now get in your bed!!
Heidi:  DARN IT!!  

I think she was quite surprised that without seeing her, I knew exactly where and and what she was doing.  Besides, I could hear her voice echoing through the heater ducts.