[shorod]

Not necessarily. There will be several devices wanting to draw current, and many of them will have capacitors inside, which after being unpowered for several hours are now discharged. The voltage you are reading may just be due to these discharged capacitors.

If you have an old test light with a bulb instead of LEDs, temporarily connect it in place of your multimeter and see if it starts out glowing brightly, then gradually dims (as the capacitors charge). If so, and it eventually gets to a point where the light is really dim or possibly even out, you should be safe to reconnect the fuse box to the battery.

-Rod