The only thing "standby" mode does is turn the video outputs off.
That's it. Other than that, the TiVo is still doing everything it does when not in standby mode.
The only exception I know of is that when the unit is in standby mode, Emergency Alert Service broadcast messages will not interrupt a recording by scrolling a message across the top of the screen.
Philips Series 1 HDR212, Nov 2001, never activated (retired)
Original OLED Series 3 (Lifetime) - Nov 2006, upgraded to 1TB in 2009
Premiere Elite/XL4 (Lifetime) - Oct 2011