User blog comment:FairyRaven/Fairy Tail Chapter 406: The Girl Within the Crystal, Review and Commentary/@comment-24536822-20141027163534/@comment-7418318-20141028001622

I wouldn't be surprised if they had left the scarf jammed into a shelf or just thrown it into a corner, if they thought it was just a piece of clothing.

Or maybe one of the soldiers took it for himself and stashed it in his quaters (I doubt they'd be allowed to wear it on duty over their uniform... it'd kinda defeat the purpose of having a uniform if they are allowed to wear stuff over it).

Either way the crash probably landed it in a heap of rubble. Though considering the size of Plutogrim it is quite a big coincidence for Gray to just happen across it... but then not any more than Lucy coincidentally landing at the place where F-Lucy's journal ended up (unless someone helped with that a bit).