I went back down today, much earlier in the afternoon having read Kris's post, and sure enough the ravens were on the roof of the building below the stack - 3 on the NE corner flapping and croaking and generally having a great time, one up on the second little landing above the roof wandering around, croaking down to its pals and hopping up and down on the railing, and 1 sitting quietly over on the SE edge of the roof beside a little wall...........perhaps Mom with one eye open watching her chicks while she had a little break?
So - 5 out of 6.........they haven't gone after all!! Seems that the trick is to go earlier than I did yesterday when there was no sign of them anywhere.