What are the small white gate type things for that you see in fields against walls and fences. you can't go through them so they must mark something. Is it a cable or pipe buried underneath?

There is a lot less of them than there used to be, some have a notice on them that says 'this is not a footpath'.

Can't say I've really noticed any myself.

Can you give us the location of one or two so people could check them out?