Why איפה אנחנו is enough in this case? I'd have written איפה שאנחנו, would it be right?
I feel like something needs to go after אנחנו. Maybe this is my Englishness showing, but could we say נמצאים or something like that?
I think you can say נמצאים but it's not necessary
כן, גם אני מסכימה
Who let them out and why are they after us?
the word נימצאים should be accepted as the last word in this sentence and not considered an error:
הדובים חופשיים והם יודעים איפה אנחנו נימצאים.
Your sentence is now accepted.
Is "אנו" in : "הדובים חופשיים והם יודעים איפה "אנו really wrong ?
No it just hasn't been added yet. אנחנו and אנו are synonyms.
Why doesn't חופשיים have 1 "י" ?
The singular already has a yod: חופשי; thus, suffixing -ים results in two consecutive yods: חופשיים.
The bears are free to eat whatever is available!