MDS777
The name's Riley
The most unbelievable thing just happened. I was walking through Route 104 to head for the pokémon center, when the Dex Nav picked up a Wingull; however, this time the search symbol had a red circle with an exclamation mark in the center. I looked up the information for the Wingull and not only did it know an egg move (Aqua Ring), but it was at level 15 too. Though I lost 2 of my pokémon battling it, luckily I was able to catch it. According to the Dex Nav, I encountered Wingull 12 times before catching this one, so I think that when you encounter the same pokémon at least 10 times, there's a chance it'll be at a higher level than normal
You don't need to see it x amount of times. I watched a guy doing a playthrough and he immediately found a level 15 Taillow with Boomburst in Petalburg Woods.