I know how annoying it is. My Sophie comes to the office with me and barks at everything all the time and I am getting alot of crud over it.
She even barks at home when someone enters the room wearing something different, or carrying a purse, or a box or whatever. I call it the "Alert the media" bark.
At the office it doesn't make sense for me to yell or make more noise when she does it, as we just look like we are completely nutz. I am at my wits end. I keep my door shut and play my iPod to block extraneous noise. I will try the scarey can idea at home.