First there would either be a prompt when you drop an item ("Are you relinquishing ownership of this item?"), or perhaps there would be a command (/place). Either of these would allow you to set an item on the ground with it temporarily unable to be picked up by other players. In sense it is still in your inventory, but others can see it and perhaps do things like look at it, but may not pick it up and therefore transfer ownership.
To prevent random spaming you can only leave it out for X amount of time, and then it returns to your inventory. You may also only walk X distance away from it before you forfeit your right to it (Or it returns to you inventory). Perhaps to keep you from accidentally walking too far away, there would be a flashing icon to warn you that you are getting too far, it starts out slow, but gets faster the further you get away. Then perhapes a system message pops up asking you if you want to give up your item. Also, to protect against the game crashing and you losing your item, if you client crashes, the server simply returns the item to your inventory.
'Placed' items would also have a new button labeled 'release' or something, clicking this will que a prompt, clicking yes will release ownership of the item.
you shouldn't drop stuff you're not willing to lose.
<shortcut6 name="next item">/target next item</shortcut6>
<shortcut7 name="pickup"><![CDATA[/pickup
]]></shortcut7> although they don't look quite like that in the shortcut interface.
meaning someone else picked it up in the crowd of miners.
As a side note why not alloy people to be able to hold anything in their hand/s. I mean if it can fit in your inventory (in bag on your back) you should be strong enough to hold it in your arms. I know this can already by done with ores and stock. Then instead of dropping things on the ground where you can noobs and jerks run off with it, allow players to see what other people are holding i.e. right click on the item in their hands. Alternatively work that into the description window. This would also mean you can check out peoples weapons which you would be able to in real-life - differentiate between iron short sword (dangerous) and maybe a fire short sword (not so dangerous). Also means people can intimidate others without actually fighting for some fun RP.
As a side note this would also fix the un-able to equip armor to repair it like the repair weapon method :P
This would also solve the problem of things being littered and players loosing stuff on a crash.
*chuckles*You forgot to add:
Facing the item is -key-, and reduces the chances of going undetected to near zero. If the theft is a success, and no one was aware of it, no messages are given. However, anyone who -did- see it gets this: "<thief’s name> is trying to take <owner>'s <item>!Not sure if physically facing would work out well, even though it obviously would be realistic. The characters can't face things as easily as one can IRL, and one has to constantly readjust the position already, so it might become very tedious. Possibly the intent, but it could render the system unmanagable. Maybe it would be better to have the owner auto-watch the item placed, and anyone can optionally watch any item anywhere. There needs to be a limit to how many items can be watched at once, as well as the distance, and the line of sight plus size and awareness and such.