Depth Perception?

Until recently I thought of my daughter as fearless. Lately she has developed seemingly odd fears of cracks in the ground. We live on the fourth floor of an apartment building, thank God we have an elevator. The last few times we have gone in or out of the elevator she has cried unless someone holds her hand when crossing the gap in the floor where the doors open. She is petrified of it. I guess since you can see down it that would be a rational fear especially for a toddler. Her fear of it is not what has surprised me it is the sudden onset of this fear.

She also recently was afraid to walk on a friends deck. We attend a play group at the local library every week. In the room there is a large stage that is put together by 8 separate squares being joined underneath. The stage is only a foot or two of the ground. Today she was terrified to walk from one square to the other. She wouldn't cross the cracks unless I held her hand. She tried to get an older girl, possibly 8, to hold her hand once to help her, but the girl didn't know her and didn't understand. The thing about this is last Monday she didn't even seem to notice the cracks.

I don't know if this is a normal part of development. Has she suddenly acquired depth perception? Or is she just more aware of these things? I know nothing bad has happened to her in conjunction with these cracks.

I guess she just recently realized that part of the floor is different, but she doesn't know why. Maybe that is why she is afraid because she doesn't know what it is. I don't know, and I'm not sure if I should be concerned or not. It's no big deal to reach down and hold her hand, and to reassure her that everything is alright. This is just all so new to me.