Usually when in back of is used in English it is in reference to a fixed location which has a permanent back and front area.
EG: it is recommended that utility meters not be placed in back of residences as that requires meter readers to access the back yard of houses. The area closest to the street is generally accepted as the front of the property and the area on the opposite side of the structure is regarded as the back of the property. In back of the house never changes while behind the house depends on the layout of its architecture. It is not common but possible to be in the rear of the house while on the side of the property. But the literal back of the house is always the farthest from the street.
Elephants always move so you refer to yourself as being behind the animal.