What happens to spent developer left in open container?

A two-part question, as I may move into exploring more developers, and also try to understand disposal better, adopt best practices:

What has happened to the spent typical (modern) developer?
Have strong compounds like hydroquinone changed in the process?

What happens to spent developer left exposed to air, sun, for an extended time?
Is oxidization or other change taking place, and is it useful for the purpose of disposal?

In an open container they oxidize by reaction with air. The long answer is more expensive and best covered in "Photographic Processing Chemistry" by LFA Mason who was head of research at Ilford in the 70s.
In a closed container, nobody knows except for a few compounds. I don't think it is very profitable to research.
It is commonly thought that the small amounts of disposal by hobbyists are not significant but in the days when there were larger commercial users manufacturers were encouraged to seek alternative compounds that are non toxic to aquatic organisms.