First off how many people did you give lv4 RC? If this person is smart enough to do this to you, isn't it a logical assumption he also deleted any log references to himself/herself? It will of course still show them logging out but the rest could have been deleted so there will be no proof other than what is on the master logs of Graal.
Limit your PW to just one Lv4 RC (yours) with all the rights and you will not have this problem.
Added: Since you are under development there is no need for more than one Lv4 RC period. Rights to change accounts, server options and the like are only needed by the Owner/Manager till such time you are going to be visible, then and only then should you have more than one Lv4 RC and only by a most trusted person on Staff. It is my opinion you should never have more than 2. Servers with more than 2 Managers are just asking for trouble in more ways than one. Asst. Managers should not have the same rights as the Managers, another problem I know of that goes on. Think before you put your PW in jeopardy, use common sense when administering rights. If your IP ranges have not been put in as suggested , then it may not have even been someone of your Staff. The rule for IP ranges was made for a good reason and I strongly suggest you follow it in the future.