When a Spark users sets his status to invisible, on another Spark client a status icon correctly switches to offline 'gray' icon and the contact drops from the connected users part of a group to the bottom of the group. But it also gets an "- Offline" text status. Which lets more sophisticated users to guess, that he is not actually offline, but just switched to the invisible mode.
This is not happening when someone logins as invisible. So it only adds this when a status is changing from Online to Offline. May be related to how Spark handles different than Online statuses with different languages. If one is using not an English GUI language, then Spark is adding text statuses for online users if they use some other language and Online is translated to some other word, like "Prisijunges". Normally Spark is not showing text status for Online, but it will show this translated value for every online user in your list if you are not using the same language GUI as they.