It's been the culprit in these forums quite a few times before due to overzealous rules, often coming through one of the blacklist subscriptions. Typically they try to block advertising images and block more than they bargained for. (Those are actually images, not buttons.) NoScript wouldn't apply as they're not inserted using JavaScript.
Try temporarily disabling ABP and see if they're restored. If so, try re-enabling but disabling any ABP subscriptions you have to see if it's their rules or your own causing the issue. Also, try a Shift + Refresh to force your browser to download a new copy of the images; the occasional cache glitch can also be to blame.
Hope this helps,
John

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