These numbers are roughly consistent with steaming of Netflix or Hulu at standard definition based on the hours and data used that you posted.
As advised change the password on the MIFI, however you need to change two passwords: The first is the one you use to manage the MIFI device ie to look at usage etc. The other password is the WIFI Pre Shared Key (PSK), this will be in the WIFI section when you login to the MIFI device, this is the password your laptops etc use to connect to your MIFI. You should change both of these immediately and make them complex and hard to guess ie do not use your Dogs name.
Although it is possible that someone could use a directional antenna to access it, over a quarter mile it would take some skill that your average end user would not likely have. If your neighbor at the 1/4 mile mark is in IT, I would be concerned, if they are a retired farmer, I doubt it very much. If it is another neighbor that is farther away, it takes truly heroic effort, clear line of sight and higher end equipment to be able to get a signal stable enough for streaming once you get past about the one mile mark. The other thing is that my experience with using directional antennas over long range, streaming is not very stable and it may not be worth watching it under these circumstances.
A simple way to disrupt a potential data thief is that WIFI signal is very susceptible to obstructions. If you have a basement, try moving the MIFI downstairs, then they do not have line of sight. Also sometimes it can be as simple as putting an obstruction like a piece of tin foil up right beside the MIFI on the side of the neighbor in question or putting the MIFI in a room so the fridge or other big appliance is blocking the neighbor. It may disrupt the signal for them enough that it is not stable enough to bother with.
Also in many places data theft is a crime, check with local law enforcement, if you are certain that it was not someone in your household a crime may have taken place.
Rob
rvtechwithrvrob.com
