If it's personal ( whiskey flask inscribed to a loved one ) go look for the owner. Mobiles are sooo traceable with either looking for an " I.C.E. " ( " In case of emergency " ) number, " mum " or " home " ( just use yer bonce ).
Practically everything else ( pumps, gloves, multitools etc. ) is your fee for teaching the numpty victim not to bugger off without having a good eyeball around first and/or securing essential gear to bikes/camelbaks p1ss poorly.
I myself have grudgingly paid this fee for NOT looking around for my £200 digi camera after a crash, using velcro instead of straps and putting gloves on the saddle ( to fall off, unseen as you face forward ) instead of the handlebar.