I'm very surprised that she got home and then was seen on the bench later. However, we don't know whether she actually went into her home or, as suggested above, found she didn't have her keys so went to wait for someone at the bench (though its quite way to go from her home). News reports say that the police have her phone, which suggests she didn't take it out (which is extremely odd unless she forgot it), has two phones, or did get into the house and left it then before going out again.
Sitting on the bench suggests she was waiting for someone - either flatmates as she'd forgotten her keys, or someone she had arranged (or hoped) to meet. If she left her phone at home the whole night the arrangement to meet up seems unlikely. Though of course, she might simply have got it into her head, being very drunk, that she .TRUNCATED