Remember that Gollum would not suspect Bilbo to be aware of the invisibility powers of the Ring (which he didn't, of course) but the Ring slipped onto Bilbo's finger. Gollum rushed along the tunnel in a rage expecting to maybe catch a glimpse of Bilbo running off in the distance and altogether missed the Hobbit crouching at the side of the floor.
Plus I think Gollum did sense Bilbo as he crept nearer to him and he certainly sensed him leaping over him and running off.
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Los Ingobernables de Harlond
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