You do have a W axis, you just don't know it. It's the pallet changer. That alarm occurs if the W is not centered correctly when the pallet goes down to clamp. Other causes is vise handles left in during pallet change. Watch the tool change to see if the tool is going in smooth. Z axis is the most likely axis to be crashed and will affect the APC (w). If it has moved get someone there ASAP to re-align. You don't want to break a cam bearing on the ATC, trust me.
No alarm book here, but the W is controlled by a power mate, so thats probably a power mate alarm.