Hiding things from your player is simple from the perspective of game design. However, there is an art to it that anyone can master given a few basic guidelines. Normally, you don’t want to hide things in your game too well, but, for your easter eggs and deeply hidden secrets, these guidelines will work.

Cover

The most basic way of hiding something is to put it where the player can't see it. This is known as cover. Bury it, put a painting over it, place it under a car. Anything that keeps the player from seeing what you are hiding counts as cover.

In most video games, a hint to the location of an obscured item is given by making the object or tile that serves as the cover just a little bit different than the rest.

Disguise

Disguise is making something look like something else. The best way to do this is to make the object to be hidden take on the qualities of something else. A lumpy, brown urn or vase could be mistaken for a rock.

Similarity

If something is similar to its background, it will be taken as a part of the background. This is most often referred to as camouflage, but it should be classified as similarity. Placing a bottle of super healing among a shelf of ordinary water bottles is a good example of similarity. The hidden bottle will be assumed to be just another bottle in the set.

Lesser Significance

Lesser Significance is not so much a hiding technique for game design as a detraction technique. Give the player a choice of two things, one seemingly worth more than the other. For example, two equally sized pots of which the player can only choose one. One pot contains more gold than the other, but the pot with the least gold contains a rare magic item shown as a simple necklace.