I too have had a barrel and its contents disappear from me a few minutes after placing it, but I was waiting to replicate it before I posted a report. As soon as I find a barrel on .61, I will attempt to replicate it with as much specific information as I can. If possible, I will try to test whether this is triggered by the barrel colliding with another model (tree branches, bushes, etc), and test to see if the same thing happens when placed out in the open. I doubt it will make any difference, but I suppose it's worth ruling that factor out.

However, it appears from these (admittedly anecdotal) comments here (link to DayZ subreddit comment thread) that when the barrels disappear, they are returning to where they were originally spawned, the barrel can sometimes be accessed where it was placed but it is invisible, and the contents may be (but not always) accessible from either location. Additionally, multiple people are saying that to pick up the items, you must use the ''take into hands" action, rather than dragging or double clicking the items.

This is happening still on the current 0.61 build (Nov 27th)

I did a very quick test - found a barrel in Moggy, placed it in a bush across the road from where I found it, perhaps 20-30m away then went to find food. I came back 5-10 minutes later and it was back in the shed I found it.

It seems like logging out with the barrel in your hands allows you to successfully place the barrel when you log back in.

This was on UK-04 at around 12:40am GMT