I don’t really know, but i think it was from shooting scenes on an icy world. From Wikipedia:

[…] The episode begins with a “cold opening” on an icy windswept planet. Two figures wearing survival gear beam onto the scene and search until they discover what they are looking for: Voyager, buried beneath the ice.

Plot

Fifteen years in the future, Chakotay and Harry Kim discover Voyager frozen on the surface of an ice world. They recover the body of Seven of Nine,[…]