I can't find NBT block location

I want to be able to assign "homes" to different entities. I don't think variables will work because they effect every entity / block. I want to have each entity of a certain type to place down an NBT block and have it's X, Y and Z coords saved to the entity for it to teleport back to at night time (or even better pathfind to it, but that's for a different forum lol.) I don't know how I can get the coords of the NBT block without using variables, which, as I said, won't work.