I think the idea is that as written, the enemy needs to take the search action to find the character and beat their Dex (Stealth) score to find them. Looking behind a rock is a sensible narration of that check, though it might fail - failure might be narrated something like this scene from Two Towers :
If the enemy doesn't take a Search action, then as written, it doesn't seem that they can "find" an invisible target. "Somehow seen" would refer to a successful Search action, breaking the Concealment benefit of the Invisible condition, allowing them to be seen and therefore ending the Invisible condition as per the Hide action.
The roughness here is, I suspect, at least partly down to what Line of Sight means and the general lack of facing in 3.5+e D&D. It's clearly possible to sneak up on someone despite being in their (D&D) line of sight (by walking up behind them), but an interpretation of the Hide/Invisible interaction that means that breaking cover means you are automatically spotted would prevent this, which clearly limits it's usefulness. Even stepping out "in front" of someone may not (realistically) cause you to be seen - I'm sure we've all walked into someone while distracted by something.