This special encounter is only available after restoring Reborn City.
Within Rhodochrine Jungle, there are several invisible Kecleon. One is in a cave, blocking you from reaching a Treecko that is trapped there. Inspect the Kecleon in the cave to begin the quest. After doing so, there will be a civilian talking to a staff member in the Silph Co. building in the Obsidia Ward. After the civilian leaves, talk to the staff member. She tells how residents of the Beryl Ward are troubled by the invisible Kecleon and ask if you are there for the same reason (your answer does not matter). She then asks you to steal a Devon Scope for Silph to reverse-engineer.
Go to the top of the Devon Corp building. There you'll battle Street Rat Alphonso (he has three Pokémon, level 75–77). After defeating him, you can grab the Devon Scope Model along with an Electric Memory and TM82 (Dragon Tail). Return to the Silph Co. building with the Devon Scope Model, and you will be rewarded with the Silvon Scope.
Return to the cave in Rhodocrine Jungle and inspect the invisible Kecleon again. Using the Silvon Scope will reveal a large horde of Kecleon that you must defeat in a Double Battle. Afterward, a purple Kecleon will appear, revealed to be a bully that goaded the other Kecleon into hiding. Defeat the bully Kecleon (it's stronger than the others), then use the lever to open the cage. You will then have access to TM33 (Reflect) and Treecko, who will join you without a battle.