This is most likely a reflection from glass between the phone and the scene, such as a window or barrier. The camera captured both the scene beyond the glass and a faint reflection of the phone on the glass surface.

It was probably only noticeable because the phone was brightly lit at the time, likely by sunlight coming from the upper-right. When the camera/phone is illuminated, its reflection in the glass becomes much easier to see.

That’s why it may appear in only some shots from the same location: a small change in angle, lighting, or position can make the reflection stronger or weaker.

To reduce this kind of reflection, move closer to the glass, change your shooting angle, or shade the phone with your hand or another object without letting that object enter the frame.