Live in new build estate. You know the sort with houses and flats, some driveways, some on street parking and a lot of spaces with numbers corresponding to house/flats.
There are also a few visitor spaces in each car park area.
I live in a block of 4 flats. Only 2 of us have cars. 2 of us had an agreement with my old neighbour our visitors could use his space when they came - unless he had visitors (which was rare!)
Some neighbours obviously cottoned onto to the fact the space was empty overnight and cars began appearing on driveway at nighttime but always gone for whole day - I expect they are at work.
New neighbour moved in a few weeks ago. They are doing up the flat before moving permanently. He doesn't have a car but his family do and are here most days helping with decorating etc.
Recently, past week or so, cars have been using their space and leaving car there for 2/3 days without going out. Neighbour asked me if space was there's and I said yes everyone had allocated space. They asked because agent said the space wasn't part of the flat. Maybe they are owned by grounds company? Who knows? But it clearly has the number on it - and what's the point of numbering them if they intend for people to park where the feck they like!
So I speak to new neighbour yesterday and say I'm happy to park in their space and they use mine when they come - I said i didn't think anyone would use my space as they know it's occupied daily. I did this and car parks in my space overnight and so neighbour still couldn't use her space when she came today.
Been out all afternoon and when I came back both spaces are empty - so I've parked over them both 
AIBU to do this to prevent some random dumping their car there this evening and possibly not moving it again for 3 days? I'm out early tomorrow for whole day trip with DS so I know I'm not preventing new neighbour using her space when she comes.