If she thinks she has got away with it this time it, it will just happen every time she comes to your house.
I had this with my SD, started small things, i called her out on it and asked why she took it and DP said it shouldn't matter it was just tat we didn't use & phone chargers (he feels guilty he left her with her mother and she can do no wrong). It got prolifically worse after that. DS computer games, but I couldn't prove it. Costume Jewellery, clothes, shoes, money, underwear ffs, she used to go through everyone of my drawers, you know when you can tell someone has had a rummage, she used to go through stuff I tried to stash under the bed. She always denied it and i could never find any proof, and if I did she would find an excuse, or it wasn't big enough to warrant her a telling off from daddy - I did very nearly leave him. Worst part was when I started having to take my handbag to bed with me. If I left my purse out and she called round unexpectedly she would help herself to a few quid. I had a money jar hidden in the corner of our room behind some drawers and curtains, I don't know how she managed to find it but she did, I went to put something in it and it only had small change left in it instead of £1 and 50p, I could prove she did that though as there was a ring in it and i found it in her things. I got lockable make up cases and vanity cases for all my stuff.
She is a lot older now and has matured, early 20s, I don't think she would take stuff now but I am a bit on edge if she ever comes around and goes upstairs to us the loo. But to be honest now she is older I wouldn't pussy foot around her to keep the peace, she would be told outright.
Nip it in the bud, my SD made my life hell for a good 8 years, but she had issues at her mums and DP was reluctant to discipline her - he now knows that backfired.