Board Thread:General Discussion/@comment-90.6.9.101-20140305214909/@comment-112155-20140314114550

Nah, the most probable option is that Sif don't know all the events that happened in Earth in the millenium between that passed since the invasion of the Frost Giants and the events of Thor.

Besides, the alien could have visited Earth more than 1000 years ago, and be founded in suspended animation.