I have a 3225 and don't know what that hole is for either (nor is it addressed in the manual)(Any responders - it's not the reset hole).
But, make sure you go to:
to learn how to upgrade your system with 6 Million POIs vice the 1.3 Million it currently has...
I purchased a Magellan Maestro 3225 from Costco for $149.99. When I opened the package, I noticed that the unit has kind of a microphone on the side of the screen (a little tiny hole). Any one knows the purpose of that? I called Magellan and I was told that it is for the future update (voice command). FYI, they're also working on an update for the anti-theft option (a PIN).