Mystery AVHD file and no snapshots lists in Hyper-v

I was the victim of the runaway shapshot eating all the availble disk space last year - that was a fun 30 hours to fix.  I still hold the record of having a 400GB snapshot.  Snapshot are evil in production - never to be used again.

Anyway...

I have one virtualized exchange server and in Hyper-V mangare, there is no snapshots listed (this is good, since it is an Exchange server and it is in production).

But, we are running out of diskpace again.  I check the drive and there is a 151GB snapshot (which is active and being used by the Exchange server).  A AVHD file - your basic snapshot differential file. 

So, it looks like this blight of snapshots is not completely gone.  How do I get rid of this snapshot?  

I would like some suggestions before I start working on a production system.