Hi all, just thought I would share this so that others are aware and to see if anyone else has had a similar issue. We are still trying to work out exactly what happened but this is what we think happened...
There is an Extension Attribute with TRUE / FALSE / or empty as possible values.
There is a Smart Group for machines where this Value is exactly TRUE.
When the Extension Attribute name was shortened the madness began! Basically a whole bunch of machines then became members of the Smart Group and had policies applied as a result. The machines then appear to slowly over time fall back out of the group as the system realises they aren't meant to be there.
Whats odd though is that everything you look at correctly reflects the new name and the machines show the value of this as FALSE, as they did with the previous name, but they still show in the membership of the group.
I can't think of any logic that would put them in the group but that certainly looks like what has happened, if they had fallen out of the group it would make some sense as maybe they no longer met the condition but they shouldn't be moving into the group!
In the end I did fix it, simply by clicking edit then save on the smart group and it immediately updated the group memebership to the correct machines!
Anyway just be careful when renaming EA's I guess...