The suface book uses the base battery for the majority of the drain while using as a laptop but then as the base battery gets low it starts to use more of the tablet battery while still using some of the base battery. If you used the laptop all day you must have depleted just about all of the base battery causing the system to use the tablet battery to keep running. When you detached the tablet and had 30 percent that was all that was left of the tablet battery and im sure most of the base battery was depleted.