There is no way to "lose" your money randomly. No one can take it unless you pay them with /pay. You could've possibly clicked on what's called a "Trade sign" which give you materials in return for a certain amount of money. :)
Poaching is not possible. Is it a possibillity that you had someone in real life over that went on your account while you were afk?

 

Did you shout at all? Shouts cost $1,000 each, so this could have been the issue. Money doesn't disappear, so you may have accidentally bought something from a trade sign, too.

 

It looks like your friend took it, in the future log out of your minecraft completely before going afk. :p There isn't anyway to get logs of who you paid sadly. Although it's possible to get logs, the staff on the server is too busy to go through days of logs to find it, sorry. :p Just like I said, log out all the way (meaning to the launch screen) and be sure you never tell anyone your password, either. Have fun. :)

 

This happened to me awhile ago while I was going through a portal. I randomly just lost 1k. I don't shout(it's a waste of $) there were no trade signs and 1k just got taken. Some random very nice president kindly gave me 1k