Seems to be down to a Minecraft bug.

My test case:
Place a glass block in front of a sign which controls a portal.
Click the glass block and onBlockDestroy triggers for the box.
Destroy the glass block and once its status is set to 3 (destroyed) we also get an additional event for the sign, with an additional status of 3. The type of that object thrown is a sign, and the positioning is the same as the sign. The game considers the sign to be destroyed for that split second.

Not sure how to handle this. We can't check to see if the sign is still there because even if we destroy the sign it's still there for the remainder of that event, with a status of 3.