A = [1, 2, 4, 5, 7]
  A_s = sorted(A)
  b = A_s[0]
  for a in A_s[1:]:
    if a - b > 1:
      print b + 1
      break
    else:
      b = a
  else:
    print 'No missing element'

Trying to be more Pythonic:

  def test(l):
    l_s = sorted(l)
    print [l0 + 1 for l1, l0 in zip(l_s[1:], l_s[:-1]) if l1 - l0 > 1]