Witnessed an item source GUID today. Source for SPELL_AURA_APPLIED applying gun scope proc. Destination player unit.
http://www.wowhead.com/item=77529
GUID: "Item:3299:0:40000000476C13EC" (3299 being my realm ID)
Name: "Unknown"
Flags: 16936
Spell ID: 109085 (
http://www.wowhead.com/spell=109085)
Edit: Don't know if this is new actually? Come to think of it, wasn't there something introduced in MoP?