After a week of national concern (and just the teeniest bit of schadenfreude), Tinkerbell the Chihuahua was returned to her owner, menace to society Paris Hilton, on Aug. 18. But where had the armpit accessory gone? Was her disappearance a publicity stunt for the unauthorized ''Tinkerbell Hilton Diaries'' (Sept. 1)? Dognapping by jealous comeback canine Benji? Or just Tink's desperate attempt to flee her overexposed mistress? ''I'd gnaw off my own leg to get out of there,'' says one schnauzer who prefers to remain anonymous. ''You lie down with trashy heiresses, you get up with fleas, know what I mean?''