The ONLY reason that I would EVER leave my cell phone behind is if I had known and trusted the person. If some of them left all of their possession's behind (purse, phone, etc) it tells me that they thought they were meeting someone (that they knew) on the fly. Maybe for drugs or a loan? Many times, these men will offer to help financially. They were all easily lured and did not stick to their usual protocol. For someone in their profession, who is usually on high alert at all times, knowing what COULD happen, I am convinced that the SK got to know these girls and had met them all before he killed them.
I agree with you completely on this, AC_Lucky1. Building trust, especially a familiarity with their routines, perhaps appealing to their needs to build that trust, enticing with access or rides even, then finally luring at just the right moment, alone, unseen, off their guard, the final time, all seems possible.
If something like this did happen, it struck me that considering how well it may have been planned for, then would he use his own car? Could he have been using a rental on these occasions to minimize his risks? If he did use a rental, I imagine it wouldn't have been from LI, as that would've been easier to trace with corresponding dates, so could he have rented cars in CT, NJ or NYC even, for initially grabbing the women, then his own car, later to dispose of their remains. It would make sense that he wouldn't want to be seen with them alive and recognized, if he's familiar with the area.
Then there's this whole thing of selecting the women. Could he have been canvassing or scoping, getting to know many women, then focusing on the ones that he could grab most easily, like a salesman would approach making sales?