If your fob is buried deep enough in a bag with enough junk surrounding it, the car won't be able to read the RFID from the fob. You can test this if you take your fob and put it in an ESD bag (like what EZPass gives you or computer parts come in) or wrap it in tinfoil. I promise you its possible to lock it in the trunk. Under normal conditions with a new battery its probably highly unlikely but trust me I've done it on another car. Its not like Ford has black magic when it comes to key fobs, its just basic interference.
