"It means you can't tell, because it has no sign."
In a nutshell.
It means that the milk comes from farms, not all, possibly some farms, and most likely many farms, that treat their cows with rBGH and the higher levels of antibiotics necessary to offset the infections in engorged cow udders that are in a constant cycle of providing milk.
So, yes, milk without the "no rBGH" labeling info does indeed contain it.
Just to clarify, rBGH (bovine recombinant growth) is a growth hormone given to the cows so they produce more milk. Its use also requires the use of additional antibiotics. While the FDA maintains that rBGH (and rBGT) are safe, it is banned from use in Canada, Australia, New Zealand, Japan, and all European Union countries.
Some states restrict or prohibit the use of the "rBGH/rBGT-free" and/or "antibiotic-free" labeling, so not seeing the designation doesn't mean the milk contains the additives, but, as others have said, it's a good bet they do.
I don't know if it is really safe or not, and I don't want to start a spirited discussion on the subject. For me, however, less is better. If I can buy milk products that don't contain the added ingredients that's what I buy.