You are Missing A smith and Wesson 357 cal model 686 with registration number AWU7752?

The best thing would be to report it as stolen (if you suspect it was stolen) or missing to you rlocal law enforcement agency. That way it can be entered in the national databse of stolen/missing firearms. If, at a later date, it should ever be offerred for sale or registration, when it is checked against the databse it will show as 'flagged.' ALSO - if this can be traced to you by the serial number, reporting it will relieve you of any responsibility if it should ever be used in the future in the commission of a crime.