Q: Mysterious startup restore on MacBook Pro without battery

Hi guys,

I have a pretty old MacBook Pro 17", Core 2 Duo. It works without battery, always plugged into an electric outlet. Sometimes I occasionally disconnect it so it immediately shuts down and any current work is lost. BUT there were a couple of times for over 4 years when after such disconnect and a restart MacBook didn't run the standard startup procedure, but instead did show me a gray login screen with some kind of progress bar at the bottom of the screen. Right after I press the power button. No Apple logo, just a login screen. And after a couple of minutes when login screen becomes colored and active, I login to my MacBook and see all apps and all windows and all work, even unsaved documents, already restored, just like it was not shut down but instead was sleeping. But I want to repeat - it doesn't have a battery at all, so it can't actually sleep when not connected to the power outlet. And last time (which happened just today) it was unplugged for a day and when I plugged it in and turned it on, everything was like I just described.

What is this? How does it work? How can I force it to always behave this way?