Monitoring Logged In Users in MS Access 2016 - vba

Question

I have a form on a tab that I use to monitor users who are logged in to my application.
The table is called USysRollCall (prefixed with USys to automatically make it invisible as a system table if access is gained to the navigation pane)
Only 3 fields are necessary: UserName (string) , As_of (date) and Logged_In (boolean)
All new users are automatically added to the UsysRollCall table
A "frm_Invisible" form launches with my main form and remains in the background. On the load and unload event of this form, I call the following routine like this:
Upon logging in: UserActive(AcquireUser, True)
Upon Logging Out UserActive(AcquireUser, False)
Public Sub UserActive(strUserName As String, bTrueFalse)
If bTrueFalse = True Then
CurrentDb.Execute "UPDATE USysRollCall SET Logged_In =-1, As_of=#" & Now & "# WHERE UserName='" & strUserName & "';"
Else
CurrentDb.Execute "UPDATE USysRollCall SET Logged_In =0, As_of=#" & Now & "# WHERE UserName='" & strUserName & "';"
End If
End Sub
'The following function can be used any time to get the username. It also serves a dual purpose of automatically taking necessary actions to add a new username to the user table if it is called and a username is not in the table. (not provided for this example)
Public Function AcquireUser() As String
'Returns the username
'but checks to see if it is in the table - if not... it adds it to the table as the lowest security level
Dim strUserName As String
strUserName = GetUserName
If Len(CheckUser(strUserName)) = 0 Then
MsgBox "Will be adding " & strUserName & " to the database"
AddUser (strUserName)
AcquireUser = strUserName
'MsgBox GreetUser(strUSERNAME), vbInformation, "Welcome"
Else
AcquireUser = strUserName
End If
End Function
I can verify that when I login and check the backend table, UsysRollCall I see a check in the Logged_In box. I can also verify that no matter how I close the database i.e. appropriately by using my quit button - or even just using the close X in the top right, that it still effectively logs me out.
I cannot verify what causes people to remain "Logged_In"
Is there a piece I am missing?