As has been said before, no one really knows, but my own testing suggests its the alarm module warning of a low battery condition (<9.5v on start, <11.5v ignition on), which could also indicate a wiring problem (poor connection). Every time my car did it (with the previously dodgy battery) one or more of the following ECUs showed a DTC for 'Low battery volts':
Central Locking or Radio or Instrument Panel
00668 Vehicle voltage, terminal 30, Signal too small
Ultrasonic Monitor:
01462 ATA ultra sonic sensor -G209- Power supply Too small/Too large
Airbag Module:
00532 Supply voltage too low
(NOTE: don't VAGCOM the airbag module unless u know exactly what u are doing.... rumour is you can fire the airbags unexpectedly)
But there's not hard documentation to say that this is cause and effect