I agree, I doubt it was a first aid kit unless it was on old empty one used for this purpose, even then they are not very big.
First aid kits are sealed,serialized and tagged with expiry date sickers or equivalent. It is a required piece of equipment for flight.
In my experience with over 5 airlines the flight attendant's are very quick to call mntc when the seal is broken or if something from the kit was used.
As well all first aid kit's, most of which are standard and the same manufacturer from company to company would have no room in them to stack anything.