OK even the Laguna has a plastic safety catch ( pictures to follow )
The main catch is the stem with spring ( underside of bonnet ) which fits into a circular opening on the engine compartment front member. THIS catch is locked by a plate which moves back and forth. THIS plate MUST be regularily greased to ensure it's free movement to it's return locked position.
The plastic catch will allow the bonnet to remian secure upto 30 - 40 mph but you will notice the bonnet flapping around if the main catch has not locked.
Hope this helps - Although the subject has been raised by Watchdog I do not think it's a major issue ( provided that you ensure you bonnet does lock down )
Hope this helps