My dad has experianced this issue with his key. I haven't experianced this issue using my key. I appears that stroking the key a few times against a down jacket so it becomes static solve the issue. I can't explain it though. Why would the immobilizer unlock itself suddenly by making the key static? I recall my dad has partially solved this issue in the past by cleaning the key with a cotton rag, some times it works, some times it doesn't.
I litterarly tried to start the fabia today using my dads key and I got the immobilizer icon flashing on the dashboard. I took the key and stroked it a few times against my jacket and the car started up several times after than with no icon flashing.
Also when I think about it, the central door lock doesn't work either maybe these two issues are related.