Yes, the audio signature background noise on the CVR would be very significant. Having heard 100's of hours of cockpit recordings, I find unusual cue sounds are pretty easy to identify. Hopefully the BEA have someone clued in on this.
Personally I am surprised the Captain choose to take his rest when he did - knowing the ITCZ was coming up and the weather less than optimal. However I have no knowledge whether this was Air France normal practice nor do I know what other major carriers do.