>>Could Ida Straus have been recovered and remained unidentified? Opinions?<<
It can't be ruled out 100% and if she was, she wasn't the only one who has a headstone marked "Unknown." Personally, I think it a bit unlikely. She was a 1st class passenger and they tended to stand out a bit because of their appearal.
You'd have to reduce the list to female victims recovered but buried at sea. As the most famous female hero of the disaster, she would have been on the B.O.L.O. list and I have little doubt that, officially and unofficially, any older female corpse that even vaguely resembled her would have been closely examined. In her case, they PROBABLY had dentals and helpful medical records sent up from New York just in case.