I have been running into this problem ever since I switched to use the jenkins selenium plugin as my grid. After the grid has been running for a while, the slaves/rc servers which are connected to the grid start to appear as "in use". When I logged on to the rc servers to check whether they are actually "in use", they were actually idle with the browser windows still open but not doing anything. I checked the RC log and it usually stuck at "Slave successfully connected":
JNLP agent connected from /10.1.104.72
<===[JENKINS REMOTING CAPACITY]===>Slave.jar version: 2.11
This is a Windows slave
Slave successfully connected and online
When this happens, it happens to all the RCs. I have to restart the jenkins server to get out of this "in use" state.