Walt goes to tell Locke that it wasn’t him who poisoned Michael. As Locke is reassuring Walt, he grabs Walt’s arm and Walt freaks out a bit and pulls away. He says “don’t open it, don’t open it Mr Locke, don’t open that thing”. But Walt doesn’t even know about the hatch.
Surprisingly when Locke first shows the hatch to Jack, Jack’s response is “how do we open it?”. It’s not everyday that Locke and Jack agree on something like that.