There isn't a hard and fast rule about carbonation or time for maturation. Going with your "assuming it's used" postulate, the secondary tank could serve several purposes. What really matters is the process between the two tanks that will decide whether or not it has any meaningful carbonation. A very common process is to centrifuge the beer on the way to the next tank. However, the "secondary," or lagering tanks, are also typically used to blend batches for consistency. So, the beer will not be highly carbonated, perhaps at around 1.5 volumes, which is enough to allow CO2 to escape during the transfer and be part of reducing oxidation risk. An American and German practice is to capture the CO2 from fermentation and reintroduce it during maturation.
With many European Pilsner companies, the beer is fermented under pressure or capped during the last stages of fermentation and transferred with an abundance of carbonation. The beer is allowed to lager to full clarity, but ages for longer periods than most western mass lagers. I'm told on good authority that Pilsner Urquell lagers for as long as 6 months. The result is a fully mature and carbonated beer, ready for unfiltered packaging but they do pasteurize for export. The same is apparently true for Guinness, with a shorter timeframe.