There is a lady who lost her key in the Valor Lakefront (Hotel Grand Lake). You need to get the Dowsing App and use it to find the key, but you need to find a certain spot to use the Dowsing Machine Poketch App to find the key.
You need to go on the side of the Hotel that has all of the houses. Use your stylus to press on the touch screen to look for the key. You should find it in the area that is highlighted yellow in the picture above. A black spot should appear if you find it. Go in front of the black spot. Press A. You should get a Suite Key. Take it back to the lady who lost her key. She will give you a White Flute for giving her the Suite Key.
The White Flute raises the likely hood of you to encounter a Wild Pokemon. I can be used multiple times.