This may be the smallest bicycle lock in the world

SmartKey can easily secure any steel Bolt and protect your equipment against tampering and/or theft
The SmartKey is a small stainless insert that fits into the already existing hex bolts, wherever you might have them screwed in. By inserting the SmartKey you block access to the hex bolt so any potential thief, equipped with a hex wrench, simply cannot unfasten the bolt and steal your parts.

The SmartKey is fastened and unfastened mechanically – with the SmartKey Key – but held in place magnetically. SmartKey can not be removed with a magnet, no matter how big or strong it is since the magnet inside the SmartKey is shielded in the “outside” direction – making the SmartKey magnetic only in the “inside” direction. That’s how SmartKey cannot be removed with a magnet from the outside but still holds in place in the bolt.