مُبْهَمٌ، وَهُوَ مَنْصُوبٌ أَيْضًا بِتَقْدِيرِ «فِي»، نَحْوُ: «جَلَسْتُ خَلْفَكَ وَأَمَامَكَ».
Translation: An unspecified place adverbial is also accusative with understood fī, e.g., I sat behind you and in front of you.
Khalf and amām specify relative positions, not particular named locations. They are the book’s examples of unspecified place adverbials.