Haiku (the pup in my avatar) can't stand to see anyone leave the house. If he thinks someone is leaving, he grabs the nearest toy and starts growling and shaking it like it's prey. This is fine (if highly annoying) when he does it with his own toys, but usually he can't find anything of his close enough. So then he'll growl and shake shoes, toddler toys, plastic cups, and occasionally DH's freeweights (which Haiku can't even lift, but he sure tries!). It's like he panics and can't do a proper freak-out unless he has something to shake. One memorable time I found him with a tiny scrap of a receipt clenched delicately between his front teeth, which he tried hard to pretend was something more ferocious.
Of course, he's gotten more perceptive about people leaving, too. When he was a pup, it was just hearing the word "Goodbye." Then he picked up on the sound of keys jingling. Now it seems like everything sets him off: keys, goodbyes, anyone putting on shoes, anyone standing up purposefully, the phrase "headed back to work" (even if it's midnight and we're in bed when DH says it!), etc. I got in the habit of giving DH a kiss before he left the house each morning - now Haiku freaks out whenever DH and I kiss
(As a result, the first step in any game of Scrabble nowadays is to put the dogs out before opening up the game box, so to speak.)
Kiki, our other mutt, has simply discovered that her secret calling is to dig through the dirty laundry, find my undergarments, and drag them outside for some fresh air
. I have to do a sweep every time the lawn guy comes.