Mello had wounded L, delivered a crushing blow to the fragile trust they'd established as a Bonded pair, and dismally failed a crucial test... and yet, L's reasons for keeping his identity as vague as possible have everything to do with a continued desire to protect him. If he ever comes back, L wants as little to do with him as possible, not foolish enough to offer him a Bond again... but not vindictive enough to hope that he meets ruin at the hands of an adversary that had ultimately ended both of them, in their world.

Of course, it goes without saying that his current priorities have gone beyond "selfish", now that he's Bonded to that adversary.

"Naturally," he replies, as though the track leading to that conclusion was a straight and uncomplicated one, and "this individual" would pose a threat to everyone as equally as he did to L, instead of disproportionately to that black and hateful grudge toward Kira's confirmed identity. "I wouldn't keep you in the dark about their potential return; to have a Bond is to protect your Bond, or... at least that's how it's meant to go."

A tiny breadcrumb of a hint, should Light choose to pursue it. Someone could certainly tell him who L's former Bonds were, and with only two before Myr, that didn't leave many suspects.

There's one loose end, of course. Near is likely to protect Mello over Light, regardless of the blond's crimes. L has little doubt that Near's general cool indifference to his predecessor extends that far.

With any luck, this will never become an issue. Perhaps if Mello returns without his memories of his time here, it would be safer... but if he's gone down that path before, it stands to reason that he could, again.

"Something to keep in mind, when you find a monster you could see yourself Bonding with."

Not that L wants to share, but he'd rather share than deal with Light exploding.