'Bachelor in Paradise' Cast Member Saw Hookup, Claims Corinne Was 'Very With It'

Corinne Olympios was topless in the pool with DeMario Jackson and so aware of her surroundings, she greeted a cast member walking by, showing no signs of distress -- but Corinne's attorney isn't buying it.

Sources close to the "Bachelor in Paradise" cast tell TMZ ... one female cast member walked right by the pool when Corinne and DeMario were hooking up, and saw her with her top off and her legs wrapped around him. We're told Corinne yelled out to the passerby by name and seemed "very with it" ... coherent.

The cast member told others she would have done something to break up the pool tryst if she felt Corinne was too drunk. Further, the cast member says cameras followed her when she walked by ... so the whole encounter should be on tape.

Corinne's attorney, Marty Singer, tells TMZ ... "If nothing inappropriate happened, why did production get shut down? Something serious clearly happened."

The female cast member says Corinne talked about the DeMario hookup the next day because she was upset her boyfriend, back home, would find out -- but she never heard Corinne say anything about it being non-consensual.