Is it Freeview's own message telling you that set needs retuning that you are getting? I seem to recall mine also had a notification, something along the lines of 'press yellow to remove this notification'.
However, my Panasonic video recorder, which is my default tuner,(ie it is providing my Freeview signal to Telly's HDMI) frequently (about 40% of occasions upon switch on) does display notice telling me "New Stations Found". And then the option to retune or ignore.
Total number of channels usually does not increase, so I guess channels which were not broadcasting when last I switched off, are being interpreted as new channels.