The A4 front floor mats are held in place by small plastic caps that are known for being lost to vacuum cleaners. The floor mat tie-downs consist of a plastic nipple that is mounted to the floor under the mat. The nipple is made to snap through a hole surronded by a plastic grommet in the floor mat. The grommet is then capped with a plastic cap that screws in place.
Many A4 owners have reported that the caps are swallowed up in the first or second vacuum job on the car. If you vacuum you A4 be very careful to note these caps, and if you take you car to a car wash service (a big no no in my book), be sure to point the caps out to the attendant and warn him of what happens to them. Your best bet is to remove the caps (they unscrew) before you do any vacuuming or plan on having anyone else do any vacuuming for you.
Part numbers for the front floor mat tie-down grommets: ZVW 178 001 (1 Piece Fastener Kit 0697 Made in Germany).