motor oil resevoir leak?

I have a 2005 mercury saltwater series motor. I'm not so familiar with the mercury brand. I am told the oil system is an air pressurized system. My problem is on the motor itself is an oil resevior that should be full all the time and has a float sensor in the cap. I filled the resevior by starting the motor and taking the cap off and the pressure filled the tank. Everything runs fine and well. The next day, go to start the motor and the sensor is beeping, I take off the cowling and see that the resevior is half empty. I do not see any visible oil leaking, especially that amount. I suspect I have an air leak somewhere? I replaced the check valve and it still happens. Anyone please enlightnen me as to what this may be. Thing runs great except I know this shouldn't run this way.

Thanks,
Chuck Brandt
LBI, NJ