Chris - that is a fantastic observation - I think it was upped recently to allow a particularly long (but stand alone) script to run and then not lowered. It still doesn't answer why all these sapphire/main scripts are persisting and eating up memory - but is most definitely something!
We're seeing the same issue here, on a site upgraded from 2.3.4 to 2.3.7. Even since this has happened memory has been spiralling up until eventually Apache restarts itself. Normal memory usage on the server is 280-320MB, it'll hit 800MB or so before dying.
lsof -p xxxxx indicates its httpdocs/sapphire. Investigating presently.
edward - /sapphire/main.php is the main file behind SilverStripe, it loads everything so that may not be the bottle neck.
The site gets on average 300 views a month but has used up 2gb of bandwidth so far this month.
Bandwidth is different to how much resources the site is using. Bandwidth is normally the files you are serving, people loading your pages, pictures... that sort of thing not usually related to the core of SS. The alerts your sysadmin is likely getting from SilverStripe could be memory limit issues which is a common issue.