Suebi unit longbow hunters has the ability to remain hidden while firing. On the field, the feature is not really working since enemy notices longbows as soon as they crawl within firing distance [simply the sight distance seems to be set longer than the firing distance]. This, of course, mutes the usefulness of the feature on the battlefield.

The fun part starts when you try longbows against defending city fortifications. As long as there are no enemy units nearby the longbows stay hidden to enemy towers and gates. They can even fire torches while hidden and take over whole sections of city defenses without losing a man.

Sounds like an exploit, then again, this is exactly the sort of thing that a stealth/special ops unit is supposed to do.