Does the object have a parent? In GMS2, children objects will show "ghost" events of the parent events they inherit, but opening them won't let you edit them as they don't actually belong to the object, but to the parent. I'm pretty sure these inherited events when visualised have that padlock (I can't open GMS2 currently to check). IIRC, there is a RMB menu option on ghosted events to open the parent for editing.