Bonnet Open Warning!!

Hi - every now and again my 60 plate mondeo goes from running perfectly to pinging up a warning that my bonnet is open even though it is not - i open and shut it several times and still the warning is there. This affects the locking of the car too as double clicking the lock button no longer flashes the lights for the immobiliser - instead it makes a dull sound and doesnt seem to have the immobiliser on although it is still locked. After a few days, give or take, this goes away and the car goes back to normal with the locks fully functional and no warnings. Cant see anything online for others with a similar problem, anyone got any suggestions?! its under warranty until september but i need to get a service done as i've gone over the recommended mileage for this currently

thanks!