Can't help you with the info you're looking for, but I used to know JWs who would plan to take their vacations in different cities from their assigned convention city, and claim to have attended there. Then attending the local convention was "optional".
Now that I think of it, it's a fantastic way to get out of attending all together. They have no way of proving that you didn't go to another convention, and if they start asking too many questions, you get to put them in their place.
Actually... why don't you put them in their place? Say something like "Excuse me, who are you, my mother?" or "Last time I checked, I was an adult and didn't need anyone checking up on me, thanks!" or "Don't you think it's rude to imply that I'm lying? I sure do." Then walk away.
You don't owe them any explanations. They didn't see you there? That's their problem, and you don't need to take ownership of their problem.