Security log 4634 shows another user logging off

Question

Security log shows users logoff that weren't even using the machine. There are no 4642 logon logs, just the 4643 logoff logs.
These user aren't even accessing another machine via the network. All machines also have no malware or virus on them.
Logon Type: 3
This event is generated when a logon session is destroyed. It may be positively correlated with a logon event using the Logon ID value. Logon IDs are only unique between reboots on the same computer.
What could be causing this?

Answer 1

It's a domain enviroment. Printers are all through a Print Server.
Below is the log of 1 such event.
Log Name:      Security
Source:        Microsoft-Windows-Security-Auditing
Date:          2014-04-04 03:04:24 PM
Event ID:      4634
Task Category: Logoff
Level:         Information
Keywords:      Audit Success
User:          N/A
Computer:      (computer name.domain)
Description:
An account was logged off.
Subject:
Security ID:
S-1-5-21-213254720-224688177-246369
Account Name:
(username)
Account Domain:
(domain)
Logon ID:
0x197EC67
Logon Type: 3
This event is generated when a logon session is destroyed. It may be positively correlated with a logon event using the Logon ID value. Logon IDs are only unique between reboots on the same computer.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Microsoft-Windows-Security-Auditing" Guid="{54849625-5478-4994-A5BA-3E3B0328C30D}" />
    <EventID>4634</EventID>
    <Version>0</Version>
    <Level>0</Level>
    <Task>12545</Task>
    <Opcode>0</Opcode>
    <Keywords>0x8020000000000000</Keywords>
    <TimeCreated SystemTime="2014-04-04T13:04:24.783747600Z" />
    <EventRecordID>108300</EventRecordID>
    <Correlation />
    <Execution ProcessID="724" ThreadID="756" />
    <Channel>Security</Channel>
    <Computer>(computer name.domain)</Computer>
    <Security />
  </System>
  <EventData>
    <Data Name="TargetUserSid">S-1-5-21-213254720-224688177-246369</Data>
    <Data Name="TargetUserName">(username)</Data>
    <Data Name="TargetDomainName">(domain)</Data>
    <Data Name="TargetLogonId">0x197ec67</Data>
    <Data Name="LogonType">3</Data>
  </EventData>
</Event>