In Leo's market: the purple stall, almost opposite to the Teufort stall; was empty, so i placed 4 chests down, then when i went to open them they were locked with LWC. In the space of time that between placing them and trying to open them there was no-one around, Also the player the Locks are owned by was not online, so...Yeah.
Yeah, as JMP says, LWC does not lock chests but the air chests occupy. If you delete a chest without removing the LWC first, the next item placed down will have the same protection that the chest did.
We really need to find a way to drum the need to unlock things into people as not many are aware of it. LWC is a great thing but it is very limited and not very bright at all.
As to the vanishing item bug: this is worrying. Obviously a new version of Bucket will bring issues with it as it is nothing more than a set of hacks and workarounds to fix Notch's god-awful code but so far I've not heard any other reports about this.