If your car has alloy wheels, then you very likely have a set of locking wheel nuts installed (see the three nuts in the image below). A special key (the long one in the image) is required in order to remove them, adding an extra layer of protection against theft.

It is important that you keep the key handy, as the mechanic will need it whenever you require a tyre change or a brake (or suspension) repair or replacement.

A wheel with a locking wheel nut installed (see centre of the wheel):