After trying the 'Parking Guard' feature on my RX300, I'm sure it is something that I'll never use. The good part is that it gives you an option to set the frame rate to 1, 2, or 5 fps. The bad news is that once set, that is the way it is until you change or disable it whether you are parked or start driving again. Of course, you also have to enable motion detection to make it only record when motion is detected, otherwise it simply keeps recording forever at the low frame rates. I've had mixed results keeping 'motion detection' enable at all times with 'parking guard' disabled. Compared with, for example, the 'park' feature on the Lukas cameras, which will automatically change from parking / motion detect mode back and forth between regular driving mode without any menu messing, the 'Parking Guard' feature seems to be next to useless. So, for short term parking it is too much of a hassle calling up the menu and changing 2 selections, then back again before you drive, and for long term parking (days) I'd worry about coming back to a dead battery.
Please, someone correct me if I've mis-interpreted the 'Parking Guard' use, I'd love to hear that I'm just using it wrong - but I don't think so.