HTC Desire :: Back Key Brings Up Search

Something fishy is going on with her phone. On the home screen when the back key is pressed it brings up the Quick Search. OK, that's no big deal. Not the end of the world.When you navigate through regular menus like Settings the back key works fine. But if, for example, you go to Contacts, Open a contact, tap the address to show it on a map, click on the map to see Street View, you would expect that the back key would back-track you through all of those steps right back to the home screen. But it doesn't. The Quick Search keeps popping up instead and it's quite a job to get back to the home screen.Reboot doesn't change that.Anyone else seen this or has any ideas? It can't be a dodgy button because the navigation works as expected in other circumstances.