You do not want to check your oil after you have run your engine for serveral minutes. Oil is pumped all through out the engine, and it takes time for all of it to run back down into the pan. If you check to soon you will not get a accurate reading of how much oil you have. And if you add oil you will be over filling the engine.
You need to check your oil with the engine off, and having sat for serveral minutes on a level surface. If the oil is low it will be bellow the hash mark area. Add oil slowly as to not over fill the engine, and remember it will take time for the oil you just put in to reach the oil pan.
As for why oil dissapears, you might have a bad pcv or you might have a small leak. I would fill the car and keep a close eye on it for the next week to find out if there is really a problem.