To reproduce:
1. Create an empty gameobject and attach a rigidbody2d to it
2. Create another gameobject and attach a box collider2d to it
3. Make the collider gameobject the child of the first gameobject
4. Remove the rigidbody2d component from the first object - the collider outline disappears

Note: deactivating/activating or making any changes to the component returns the collider outline back to normal