Detect site-wide changes

For informational/brochureware sites that aren't being updated all the time, scan these tables looking at the timestamp.

node_revisions
history
files
comments

If there is one in there that is greater then the stored value then go ahead and expire content that needs to be expired; or even do a complete flush of the site. This would be done on cron. If nothing has changed, keep the site as is; no need to expire content.