It's really not a red flag. Not knowing or having BFS memorized does not indicate that one can't implement it relatively quickly by refreshing oneself on the algorithms.

Unless, of course, you mean if all academic papers and material that describe BFS suddenly vanished and you had to come up with a solution yourself in 30 minutes with someone looking over your back. In that case, I agree, it's a red flag for whatever position you're interviewing for.