Here's some info about SQLOS and the Resource Monitor I found from Jonathan Kehayias:
As a general rule, SQL Server will use as much memory as you can give it, and it will not release the memory that it has allocated under normal operations, unless the Windows Server OS sets the memory low resource notification flag. A component of the SQLOS called the Resource Monitor monitors the QueryMemoryResourceNotification Windows Server API and when Windows sets the low memory resource notification, the SQLOS will respond by sweeping its caches internally to reduce the process working set, and release memory back to Windows.
So, if windows sets the low memory resource notification, when the Resource Monitor discovers it, is there a way for the DBA to know it too? An event notification? An alert? Something else? Ideally, I'd like to send an email when it happens.