Unexpected Milestone

Elyza can climb stairs. We've never even given her the opportunity to climb them but apparently she can I had a bunch of stuff from Costco to bring in so I set her at the bottom of the stairs (the living space we use is upstairs) and I went up and put a few things in the fridge. I figured she was ok, it's just a hall down there and nothing to hurt her or to get into. Then I hear this whining and it's really loud, louder than it would be if she were downstairs...I walk over to the stairs and she is at the top of the stairs, standing up holding onto the gate. Totally freaked me out, she could have fallen back down the stairs and I had no idea she could maneuver up the stairs... she can't even walk yet!