Hi Liam
I didn't know the answer to this; so have been doing a bit of Googling and think I might have a bit of an answer.
First up - once a device is deleted; it is gone - you'll need backups to bring it back from the dead:-
http://www.lansweeper.com/Forum/yaf_postst12418_Deleted-Assets.aspx#post44863
But that post references a new table, available since version 6 which monitors the change in devices, which should be able to show you *who* is doing this:-
http://m.lansweeper.com/Forum/yaf_postst12751findunread_List-of-who-made-changes-to-assets-or-settings-and-when.aspx#post47348
Full credit to Susan.A who posted to both of those threads
Hope that helps!
Mat