Vista - Server driver locks shared folders

I create a share on Vista or Windows 2008 SP1. No one accesses this share
from network. After some time folder for this share becomes locked.
You can see in Process Explorer that System process has opened file handles
for each shared folder.
I've traced this using Process Monitor and found that srv.sys is making
NtOpenFile call. srv.sys is Server driver responsible for file sharing.
Here is call stack in srv.sys:
SrvConfigurationThread -> InitializeServer -> SrvInitializeScavenger ->
ScavengerTimerRoutine -> ScavengerThread -> RefreshSnapShotsAlgorithm ->
SrvSnapRefreshSnapShotsForShare -> SrvSnapSetVolumeHandle ->
SrvRefreshShareRootHandle.
What is this - share snap shots? Why srv.sys is doing it?
How can I release this share root handles?