Glasses again

So Cinderella got glasses earlier this year. She hadn't had them for very long before she broke them. However by that time we had moved far enough away that we couldn't rely on the warranty issued by the optical where we got the frames. So after we had new insurance, and life had tamed down a bit after having a baby, I took her to an eye doctor here, and got a new prescription and new frames. This time I tried to be a bit smarter and went with flex frames. (The kind of frames that can bend quite a bit and go back in shape.) Well they worked pretty good for a while, slowly getting more and more bent out of shape. Then one day Cinderella left them on the floor, and accidentally stepped on them. Thankfully they didn't break, but the lens did come out. I fixed the lens and took the frames to the optical and had them bend them back into shape. They told us we were lucky that they hadn't broken, they were almost there. That was about 2 weeks ago.

Well this morning Cinderella said to me, "You're not going to like this.... My glasses are broken again."
"How are they broken?" I asked.
"The glass came out again."
"Did you step on them again?"
"No."
"Then how did they break?"
"I don't know, I just went to rub my eye last night and the glass was gone."
"Do you know where it is?"
"No."
"Do you know when you lost it?"
"No."

How in the world do you lose the lens to your glasses and not notice? So then I went into panic mode. I have no idea where that lens could be. We looked in the living room, where she was when she noticed it was gone. No lens. Yesterday, she went to preschool, and we went to the doctors office. So it could be in either of those places as well as some where here at home. I don't even know where to begin to look. So it looks like we are going to have to order a new lens.
I just wish I could get her to be more responsible with her glasses. These kinds of things wouldn't happen as easily if she would just keep the glasses on her face, except for when she is sleeping or bathing. But how do you get a 4 year old to do that?