This one can be easy to look past since it isn’t immediately obvious. You have tried walking through the security scanner, but nothing shows up when she scans you. To get pass this you need to have the bottle of vodka with the gold flakes. If you do not have this then you need o head back to the casino that you started in and go downstairs. If you are in the reception where the cloak room is walk toward the camera and it will bring you downstairs. Once downstairs go behind the bar and you will see the vodka.
Once you have the gold vodka take a drink of it and then quickly walk through the security scanner. The guard will scan you and see that there is something magnetic in your belly. She will bring you in for a strip search. You can listen to her talk for a while or just skip through it all. Tell her you want the metal detector. She will get angry and throw it out the window into the kitty litter box.
Head downstairs to the kitty litter box and walk out on the little diving board kinda thing. Equip your sythe and swing it around the litter tray a few times and eventually he will scoop out the metal detector.