screen lock suddenly appear

Lock screen suddenly appears, sometimes.
In the middle I use browser or another program.
It always happens when I lock the screen, and then releases the lock.
It never happens when I start the computer for the first time.

$ lsb_release -rd
Description: Ubuntu 14.10
Release: 14.10

$ apt-cache policy unity
unity:
  Installed: 7.3.1+14.10.20141016-0ubuntu1
  Candidate: 7.3.1+14.10.20141016-0ubuntu1
  Version table:
 *** 7.3.1+14.10.20141016-0ubuntu1 0
        500 http://archive.ubuntu.com/ubuntu/ utopic/main amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: unity 7.3.1+14.10.20141016-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-29.39-generic 3.16.7-ckt2
Uname: Linux 3.16.0-29-generic x86_64
.tmp.unity.support.test.0:

ApportVersion: 2.14.7-0ubuntu8.1
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Thu Jan 29 13:51:00 2015
DistUpgraded: Fresh install
DistroCodename: utopic
DistroVariant: ubuntu
DkmsStatus:
 virtualbox, 4.3.18, 3.16.0-25-generic, x86_64: installed
 virtualbox, 4.3.18, 3.16.0-28-generic, x86_64: installed
 virtualbox, 4.3.18, 3.16.0-29-generic, x86_64: installed
GraphicsCard:
 Intel Corporation Core Processor Integrated Graphics Controller [8086:0046] (rev 02) (prog-if 00 [VGA controller])
   Subsystem: Toshiba America Info Systems Device [1179:ff00]
InstallationDate: Installed on 2014-12-08 (51 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
MachineType: TOSHIBA Satellite L500
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-29-generic root=UUID=d0060b13-5821-4f09-9346-d66290da86d2 ro quiet splash vt.handoff=7
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 09/08/10
dmi.bios.vendor: TOSHIBA
dmi.bios.version: 1.90
dmi.board.asset.tag: Base Board Asset Tag
dmi.board.name: NSWAA
dmi.board.vendor: TOSHIBA
dmi.board.version: 1.00
dmi.chassis.asset.tag: *
dmi.chassis.type: 9
dmi.chassis.vendor: TOSHIBA
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnTOSHIBA:bvr1.90:bd09/08/10:svnTOSHIBA:pnSatelliteL500:pvrPSLS6E-00M00NH2:rvnTOSHIBA:rnNSWAA:rvr1.00:cvnTOSHIBA:ct9:cvrN/A:
dmi.product.name: Satellite L500
dmi.product.version: PSLS6E-00M00NH2
dmi.sys.vendor: TOSHIBA
version.compiz: compiz 1:0.9.12+14.10.20140918-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.56-1
version.libgl1-mesa-dri: libgl1-mesa-dri 10.3.2-0ubuntu0.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 10.3.2-0ubuntu0.1
version.xserver-xorg-core: xserver-xorg-core 2:1.16.0-1ubuntu1.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.9.0-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.4.0-2ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.914-1~exp1ubuntu4.1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.11-1ubuntu2
xserver.bootTime: Wed Jan 28 23:36:57 2015
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:
 product id 21825
 vendor SEC
xserver.version: 2:1.16.0-1ubuntu1.2