this might help to explain two issues: Where did the oil go and where is that white smoke coming from in the exhaust?
Going from a stationery non-running engine, starting up, running until hot, shutting down etc all produce variances in engine pressure. The following description I think helps explain reasons for both issues.
By nature, all four-stroke engines require ventilated crankcases. The reason is twofold. First, when an engine is started or run at low temperatures, condensation takes place and fuel vapors migrate into the crankcase. These contaminants, essentially water and raw gasoline, mix with the oil to form engine-damaging sludge. Fortunately, once the engine reaches operating temperature the water turns to steam and the fuel remnants percolate out of the oil. As long as the crankcase is vented, either by the atmosphere or by using a positive-pressure system, these nasty byproducts are free to drift out of the engine and go their merry way without causing any harm.
The second reason the crankcase needs a vent is because no matter how hard we try, there is no practical way to eliminate air from entering it. Anytime the engine isn't running, atmospheric pressure pushes air into the engine through open valves or any other convenient entry point. When the engine is running, some combustion gases normally make their way past the rings to pressurize the crankcase as well. If the crankcase wasn't vented, pressure buildup would soon cause every seal in the engine to rupture as the trapped air sought a way out.
Originally, crankcase vents were nothing more than tubes connected to valves timed to open when crankcase pressure was too high and close when crankcase pressure was too low. This allowed the bad stuff to be forced out and prevented dirt from being drawn back in. A variation on this scheme vented the crankcase through a series of baffles, accomplishing the same thing with less complication. Although they were efficient, these breather-tube ventilators were little more than open pipes spewing engine contaminants directly into the atmosphere. As you can imagine, the EPA took a dim view of the situation and demanded that manufacturers, first car and later motorcycle, do something about it.
What causes excess crankcase pressure? Excellent question, grasshopper. For starters, anything that can reduce the crankcase volume, such as overfilling it with oil, creates problems, as will a restricted or damaged breather system.
Motorcycles have what is known as a rebreather system. This system allows for the pressure inside the crankcase to remain balanced. During normal operating cycles, your motorcycle will be cold when you first start it.
After driving around, the oil becomes extremely hot. The same amount of fluid is still present in the engine just at a very different temperature which would result in different pressures inside the engine. The rebreather system allows the pressure to be vented so that it always remains balanced.
When your motorcycle has too much oil in it, oil can escape the engine through your rebreather system. On most bikes, this rebreather system will lead directly into your intake. This means that your air filter can quickly be soaked in oil. From there, oil can get past the air filter and down into your engine.
Once the oil has entered your engine, you will now start burning oil with each combustion cycle. As this continues, you will eventually ruin your spark plug. Once a spark plug gives out, a domino effect happens throughout the engine and other parts begin to fail as well. This can result in quite a bit of parts needing to be replaced and/or cleaned if left running.
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So until the pressure stabilizes and reaches equilibrium, the oil will not return to the sump and show up in the indicator bubble. If there is a pressure issue such as can be caused due to overfilling, you will end up with white smoke in the exhaust from the burning excess oil that is forced through the intake.