try rebooting or else add a Notification Area icon to the bar by right clicking and click add to panel and look for the notification area icon. I found that one time I deleted it and there seemed to be no way for it to show until i added it back in. That was a year or so ago.
Not sure what the cause was or the fix was relevant but it worked. Try it.
Oh - depedning on what theme you are using there does appear to be a little vertical blank rectangular icon in gray or whatever other color. some themes it does not appear or else is blened into the panel until it pops up as the regular icon.
on startup my lock appears - then I right click and not lock but with my theme tere is a vertical blank bar there.
Linux Mint - Pure Bliss!