These systems are from a high volume customer and I didn't want to alarm them until I had some kind of reason for the icon disappearing. I need to not only have a cure but an explanation of why and how it happened.
I scanned the system with every malware program I could find and a couple of anti-virus programs too. All came up clean. Also scanned every system in house with the same results.
A co-worker who was also working on the missing icon(s) problem got the idea to turn the clock back and when he got back beyond 3-31-2015 the icons reappeared in the Taskbar as usual. When set to 4-1-2015 they disappeared again.
My previous hasty post was two-fold. To ask if anyone had heard of a April Fool's Day malware and to give you something to try to see if your system exhibited the same symptoms.
All the cures posted on line get rid of the two registry entries IconStreams and PastIconsStream which is the fix. I then tried setting the clock back a year and ahead a year with reboots in between and the icon(s) never disappeared again.
I hope this response was more clear and I can now approach the customer with a reason and a fix.