That would piss me off. Don’t park in the same space with another bike if you don’t know the owner.
That's me, internally. It would tick me off. But externally, I'd try to act like the guys above said, to be neighborly.
I park in my own spot unless I'm riding with people then I'll share a spot.
Because no one knows what my situation is when I have to maneuver my bike, and now they've seriously inconvenienced and stressed me. They don't know my skills (conversely I don't know theirs), and they don't know my challenges or if I have a specific way I need to do things to get the bike out of the spot (and I've probably parked it specifically in order to set up the exit). If there is another bike in the spot there's now less room and someone else's property I have to be careful of. Gets worse if vehs are on all sides.
I might sound like I'm over analyzing the situation, but counting up my factors I have to account for and work with when riding or which bike I'm on, I'm now in a more difficult position to execute a simple exercise. Following that down the rabbit hole, now there's stress, inconvenience, pressure, worry about how will I accomplish this? If I could be weak and whiny for a sec, but they just made my life harder and it's hard enough already.
Like I said, maybe over analyzing, but that's my world. People can't know that unless they know me. So I extend the same thinking to others, I don't know someone's situation and I respect people's space, that includes vehicles and property too.