The filament can have absorbed the water before it was packaged. Also, 1 day in a humid environment can do it as well is my experience.

For drying, a vacuum oven is the best thing, but not everyone has one . I usually dry it at 70 Celsius / 50 mbar for 4-8 hours.

Others have put the filament at an elevated temperature for a long time with air circulation, at least above 50 Celsius if I am correct. Make sure you control the temperature well. I once tried our kitchen oven at 70 Celsius, but the temperature distribution was so poor in the oven that the reels melted and deformed, ruining the filament.

You might want to google how other people do it succesfully.