Invisible chest is there, tried it with 3 different characters, on XBox360. No sneak, no specific levels. Just travel to Dawnstar, jump over the fence, walk around behind all the houses, then to the second mine and it's under the tree (between the tree and the rock, specifically). It doesn't matter where the Khajit are either.
Havent' checked respawns yet but I've only used it once to give new characters enough gold to buy their first house. It's also a really nice source for free lockpicks.
The Khajit inventory is all there when I check the traders, so I'm thankful for the free gift. It's not hurting anything or anyone.
I hope they don't remove it from the game anytime soon because it's really nice to get a new toon set up without having to do a ton of undue running around to gain resources, and be able to get to playing the game instead. Take a carriage to Dawnstar, and off you go. Of course using it is entirely optional. Every game has its "easter eggs." I think this is just one of Skyrim's.