It's actually pretty rare for flush mounted gear to get knocked off due to the effort/time required to undo the mounting screws for the flush mounting depending on what gear it is. If you do want to make it harder again and don't mind doing it, sikaflex makes it almost impossible to get gear out of a dash without a fair bit of knife work - just be careful if it's a painted alloy dash - sika sticks to paint better than paint sticks to ally. You can either seal it into the dash or simply run a bead around the gear behind the dash panel once it's mounted up. Some punters are also paranoid about having their secret spots knocked off out of their GPS. Easiest way to prevent this is a master switch for the GPS that is either key lockable or if you have a lockable half cab - inside the cab