Previous models had an hours-run indicator and relied upon the user to arrange for a new filter to be fitted once due (typically after 2,000 hours).
Although I have not seen the 6000, I am guessing that the design may now have evolved such that the "filter" warning needs to be re-set after a new filter has been fitted. How is the re-set carried out? That must now be the next question!
Of course, the warning indicator could also be simply trying to tell you that the filter is blocked (or even missing)! If I remember rightly, the Warm-Touch used a thermistor in the air flow to determine whether enough air was still getting through.
BTW, whilst looking on line for the manual you're asking about (and without any real luck), I foolishly opened an .exe from a downloaded .rar file - and then spent the next few hours trying to clean the s*** that had installed itself on my machine. So beware! My "excuse" was that, as so often is the case, I was trying to do more than a couple of things at once. But the manual for the earlier version is available, so you could take a look at that. 
