The problem should state that there is only one peak in the sequence. Otherwise logarithmic complexity is not possible.
For example:
[1,2,1,3,4,5,0] has two peaks: 2 and 5. In such case, the search is O(n).
Yes this is something you definitely should have your interviewer clarify before start answering the question. For this problem, it says "find a peak element", instead of "find the peak element", implying that there could be multiple peak elements and you just need to return the index to one of them.