In my friend's realm, I went down into my mine (about 128 blocks from the Stronghold), normal 1x1 block stair (not stair blocks) down to the bottom , at level 8 or 9, with water running from the top and ending at the end of my staircase. My controller died and I wasn't able to rescue my character in the meantime so I suffocated and ended up at my respawning point.

Within the next 30 seconds I immediately went back down to the end of the staircase and all I was able to find was my helmet and pants, all of my other items:pickaxes, building materials, scaffolding and precious items were lost. I then went back to the house (another minute back up the stairs) I grabbed a pickaxe and a pail to retrieve the water and dig around to see if my items got pressed into a wall. I was not able to find anything. To my knowledge, there was no lava or anything else that could have destroyed all my items in the few minutes I was gone. I am certain that I was able to get back to the chunk within the limited amount of time before items despawn.