Dogs, howling, but at what?

Dogs, howling, but at what?  A little while ago, Debbie left to go “down the hill” for some groceries and to pick up some kittens we're fostering.  About five minutes after she left, our three dogs who can still hear (Race, Miki, and Lea) started a weird, mournful howling session.  This seems to happen mostly shortly after Debbie leaves, so it's plausible that they really are mourning her absence.  The sound they make is incredible – unsynchronized, inharmonious, but clearly feeding from each other's noise.  I've never been able to capture this on video (or even just audio), but I'd sure love to...