There are two ways the Navis float can start a mission without the user explicitly starting pressure activation or mission execute:
(1) Not freezing the float:
If the float is not frozen (I F command) before the communications dongle is removed, the float controller will eventually timeout.
(2) Communications dongle noise:
If the user sends the I F command, then disconnects and the communications dongle, there can be small electrical pulses as the connection is removed that may cause the float to wake back up. The float will then timeout and start a mission.  This can be prevented by adding a break switch in-line with the Navis communications dongle that can be turned off before physically disconnecting it from the float.
Both of these events are intentionally implemented as a failsafe in case a user accidentally deployed a float before activating the mission.