Was it an actual hiss, or expelling the air noisily as a huff?
Mine sometimes do it when they’re out and about, when they were smaller it sounded like a whistle, but the bigger they get, the more huffy it sounds. They seem to make this noise expelling air through their nostrils (which explains why it’s more whistley when they’re smaller).
It seems to be a vocalisation they do when they’re excited.