Jan-
Mountain lions are typically nocturnal creatures I think, except when they are wounded or sick or hungry. They did have posted warnings about them at Rocky Mountain National Park after that attack, and that area might be a more mountainous and suitable environment for them than Zion.
I had a friend who was hiking in Banff and she "stumbled" onto a grizzly bear which she and her hiking companion didn't see at all, until they got alarmingly close to it (like less than 50 feet away), as it somehow blended in with its surroundings. Fortunately the bear was preoccupied, eating some berries with its back to them, and didn't see them. The vast majority of people who get attacked by grizzly bears inadvertantly get between them and their cubs and the sows go into attack defensive mode automatically if that happens. My friend who "stumbled" onto that berry-eating bear was very luck she was not between it and the cubs, and that it didn't see her and her friend. They slowly backed away and were in awesome, heart beating fear of this large and powerful animal.
I am glad to hear you felt up for that Zion trip- I would like to go there some day as well.