Here's why:
* Arceus is not in HeartGold. Arceus was introduced in the Diamond and Pearl generation, and the HeartGold and SoulSilver games are remakes of the Gold and Silver games.
* There are no secret entrances related to Arceus in the original Gold and Silver games. The concept of Arceus being related to secret entrances is not a part of the games' lore.
If you're looking for secrets in HeartGold, you can explore the following:
* Hidden items: Some areas have hidden items you can find by using the Dowsing Machine.
* Hidden routes: There are some hidden routes you can access by using HM Cut on specific trees and rocks.
* Secret bases: You can find secret bases throughout the game, which you can decorate and use to interact with other players.
* Easter eggs: There are several Easter eggs hidden in the game, such as the famous MissingNo. glitch.
If you're looking to catch Arceus, you need to play the Diamond, Pearl, or Platinum games.