Steps:
1. Open BUG01 scene in Repo project
2. In Scene View you can see a white sprite object, but hierarchy is empty
-> If a camera is added, it is also visible in Game View
-> Build to player, it is still visible

Note:
It was a SpriteRenderer GameObject before, and the bubble_bg sprite texture is assigned to it. Could not recall what actions I have done to it to cause that. When I noticed, it has already lost the linkage with the GameObject.
But Unity should be able to clean up "ghost objects" like this.

Concern:
If I remove the texture, the sprite is no longer visible. If I add the texture back, the sprite is visible. It means these objects are hidden and this could mean that there could be more hidden objects exist in a scene.

Opening the scene with different Unity versions still able to see the lost link SpriteRenderer object:
2017.1.0b1, 2017.1.0a6, 5.6.0f3, 5.5.2f1
The object is gone when opening with 5.4.0f3 (probably due to incompatibility)