Does Time Bomb Reveal Invisible Allies?
Stealth can only be broken by a stealthed champion performing an action that breaks stealth.
Though not viable by any means, before the sunfire cape's damage was a unique passive, it was fully possible to simply buy multiple capes and follow an enemy champion around while stealthed, relying on the damage to slowly kill them.
In this case, Time Bomb reacts the same as if you cast it on a ward - because the target is stealthed, the effect won't be visible, nor will it break stealth upon detonation. The enemy would just get a sudden burst of damage with no apparent cause.