I don't really know of any real reason servers need to be able to transfer tables to themselves. I would disable this function immediately.
Along with the other comments, I would consider making transfers between employees (items or tables) require manager approval, if you think you have a theft problem. I can see the need for this, but the difference between the two is needing a willing partner in crime.
A quick (and sad) story:
We had an Aloha client who allowed servers to transfer tables between themselves. About 5 of them were involved in a little scam, and it went something like this.
Like most restaurants, servers got the beverages (soda, coffee, etc, themselves). A group of people would come in and order 4 sodas along with their food. When it came time to pay, the servers would print the check for guests. If it was cash, the servers would take the money and then split the check, and put only the sodas on the second check. They'd then cash out the bill with the food, and pocket the difference. The 4 remaing sodas stayed open on the second check. The server would then transfer the remaing sodas over to another table, and use those same sodas as drinks on the new table. When self transfer was disable, some of them got together, and would transfer the same beverages back to one another.
Who knows how long this went on, but this was a high volume business, and about 5 employees lost their jobs over it. And some of these servers had been there for years.
I'm all about giving people the benefit of the doubt-come right out and ask this server why he does this so often. Have him show you on the POS. I'm guessing his behavior will tell you most of what you need to know, and if he has no explanation, I think you have your answer.