If you really believe that you did the right thing, you would not keep wondering about the way that you handled this matter. However, you might as well stop wondering because constant ruminating over the past is not going to do you any good.
It is easy to conclude that your employee was working on a personal matter rather than doing her job when you approached, and she apparently hoped you did not notice her attempted cover-up. At the same time, there could actually be some acceptable although farfetched reasons for her to do a paper-flip. For example, perhaps she wanted to surprise you with some interesting piece of data that she is fine-tuning, or she may even be planning a surprise party for you.
Nonetheless, you are the manager, and you have a right to know what your employees are doing during the hours when they are supposed to be working. As a result, the next time she flips over a paper when you approach, you should simply say, “Hi, what are you working on?”
At that point, you will see whatever is written on the paper, and perhaps there will be some writing on the wall, too.
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