Wrinkles, from what I thought and from what Jeri said. It would appear you might as well just not use a prefix and enter it all in the subject as.

Tab: ...

If it gets used enough, you may find it gets added to the list box. The presence of the prefix in the box would appears only to serve the practical purposes of a) encouraging people to identify something as a Tab and b) helping towards consistancy of terms used when Tab is identified (otherwise you could get Tab: Tab Add: Banjo Tab:, etc.)