Bleeders can be located anywhere including a high air tube in the attic space.
Unless you have a fitting that has threads you will not be able to bleed by the old crack a thread method. High bleeder lines can also be located in the closet and various other places.
I have bleeders in my attic space that have auto vents. The auto vents also have a 1/4 inch copper line attached and if the vent sticks open you can tell by the water coming out of the copper tube sticking out of the lower roof vents.
I am telling you all of this just to show that vents can be any where. You will have to track them down.
You do not have to bleed your system every year if it has auto vents. Is the system working correctly? If so it might have auto vents but it is good to know where they are located. Remember the higher in the system a vent is the better it will work at trapping air at that location since air rises to the highest spot in the system. Good hunting!