What is the HD proximity sensor in a 2009 iMac?
I'm not 100% sure I'm understanding your question exactly, but the 2009 iMac has six temperature sensors. The HD proximity sensor refers to a sensor that's not physically connected to the HD itself, but is near the HD. 

Some software (typically utilities that control fan speeds or provide monitoring data) will use proximity sensors to get a pretty good guesstimate of the actual temperature of certain hardware components. There are various reasons why a proximity sensor may be used in addition to (or instead of) a directly connected sensor.

The aforementioned utilities will use these readings for various reasons (e.g. to determine fan speeds where the user has chosen to override the default System Management Controller (SMC) for this purpose). In most cases it's just to display the data to the end user for monitoring purposes.

In your case iStat Menus 5 is simply showing you the temperature reading of this particular sensor. I suspect the dash "-" next to it indicates that your iMac does not have a HD proximity sensor.