One month isn't a long period of time to leave a car alone...my only concern would be the gas.
Fuels these days break down in about a month or so. Over a long period, they can varnish in the intakes and cause starting problems. If you can, use up what's left in the tank and fill up with fresh fuel.
Will the cars be garaged? I hope so. Still, a car cover would be nice...one month of dust accumulation is pretty dramatic.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car