Where Exactly are Apple Diagnostics Results Stored on Disk
Have you tried?:

/Private/var/tmp

http://www.idownloadblog.com/2016/02/29/sysdiagnose-system-diagnostic-mac/

How do you get system diagnostic files from OS X?

I usually dont just post links but there are several things in this post that you can see if they are locatable on your system. If not feel free to let us know so we can search for you.