wait wait wait... you're checking your resivuar tank? Or you're opening up the cap and looking in?

The resivuar is really neither here nor there when it comes to level. It's there as an overflow, not as a place to check the level (imo). That level will always change depending on how you're running the car. If you're running it hard or it's running hot, it's going to push the excess coolant into that tank. It can then get back into the system when there's space for it. Don't look at the resivuar to check your coolant level.

Open the cap, look inside. If it's at the top, you're fine. If it's low, add coolant and check it in a few days.

hih