There a two tanks;
1- Expansion Tank (It has the coolant level sensor)
2- Overflow tank (The one exposed through the license plate cover)

The Overflow tank has the three relieve holes to release pressure in the
system DO NOT PLUG THEM!

If your Overflow tank is constantly empty you should be looking into other
places for coolant loss (i.e. burning coolant, head gasket, etc)  The
Expansion tank siphons the fluid from the overflow tank as needed.

Hope this helps,
Dan