Although the term differs from location to location, most areas recognize something called "constructive possession" of a weapon.
This is a concept that covers a lot of technicalities. Carrying a knife in a bag--if you can quickly stick your hand into the bag and get it--means you're carrying a concealed blade.
Ditto for carrying a blade in a car, within easy reach.
Not all areas recognize this, of course, and some apply it only to firearms. But ultimately, law enforcement will consider your intent. If it's in your bag because you threw it there and forgot about it, well... maybe. If it's in your bag because you want to hide a weapon there, then you're possessing.