Rear airbags
The optional rear airbags are built into the padded sides of the upright seat squabs by the rear door aperture on each side and have an embroidered "AIRBAG" tag to indicate the car has them fitted.
Were a £300 option when I bought mine and thankfully they haven't been needed to date!
I would be careful therefore if you are planning to put a rear cover on the seat, though if you left the sides clear and just covered the parts of the seat backrests that split fold, you will probably be ok.