Extrapolating that means that the DM can (and probably should) allow PC's to take 20 on skill checks if they have the time to do so and it makes sense for the skill they are using.
Sure, the DMG only mentions searching a room, but it is easy to apply the same principle to other things--like Arcana checks in this topic--to speed things along and to follow the 'Say yes' philosophy that 4th edition encourages.
Just to give my opinion on things, as has already been said your ruling is perfectly reasonable by a rules perspective. I also think it's quite reasonable to have bonus magic items that the party doesn't necesarily get unless they succeed at a skill or something like that, likr yout boots of stealth.
I don't think there is any obligation that every single item you ever put into your game world is found by the players. I make sure that enough items are easy enough to find to keep the players up to the apropriate power level, and I put a few more in that they may or may not find.