I agree. But that's exactly the problem: Why DO items show up sometimes - and not show up at others? If it is due to filters, then I'm baffled, as I have not touched any of the filters.
-J-Mac
In this scenario, I think you'ld have to explain in detail what happened - the problem is, as you say: you dont know what happened.
This happend to me at the beginning & I've no idea why but now I'm reasonably familiar with the programme it hasnt happened since so I'm presuming I did something wrong somewhere.
If you figure out how to get those items back, I think that will also help with your understanding of the programme and
maybe help you find out why they stopped showing - search will find individual items but probably better to use the date filter if they all created around the same time ( [item created] on such n such dates). When you find the items have a close look in the properties pane at what fields are filled. Possibly what happened (if they were sub-items in a grid) is that for some reason the parent link got broken. If they were top level items the source field was deselected but I cant imagine that happening.
Filters include the Date filter and the Alpha-numeric filters as well as the filter in the sourcebar
Dont know if any of that help (I'm also possibly repeating things you know...)

~~~~~~~~~~~~~~~~~~~~~~~~~~
RE all items meeting source or not -
I was happy with current situation (only top-level items meeting source) because it meant I could display hierarchy in an expected way
As far as I know if I use hierarchy display mode and some sub-items also meet source they are displayed twice (once as sub-item, once at top-level) which disturbed my sense of "order"

Correct me if I'm wrong there - I havent checked this in a long time so situation may have changed - will test it later
If I could get the display to ensure that an item only shows once (presuming it's only in grid once) I would be happy to have some sub-items meeting grid-source -as Pierre says sometimes it would be appropriate, sometimes not.