Its the gated fence and the white fence in the back corner from the looks of it…
Looks like the guy was to cheap to run the fend further up his own yard and opted for a 90* on the neighbors house .
Its without a doubt on your property, for fucks sake its basically touching your house… Is that his house to the left or is that a pool shed? Looks like you guys are in close proximity, i’d laugh if someone fucked up and he was more on your property than he thought (The concrete walkway etc)
I’d totally get a survey done