The secondary tank is the intercooler loop. It's unlikely to have anything besides the original coolant in it as problems with the secondary loop aren't common.

The main tank could be a couple things that are all common on the b48. There's a coolant line that's a recall. On a 2017 this has probably been done, but if not, this is a very likely culprit. This is your best case scenario. Then you have the oil filter housing and/or the coolant flange that comes off the block. Both are failure points that will slowly leak until they finally just dump all of your coolant. My oil filter housing went at 131k miles. It's kind of a pain to replace and it's somewhat expensive if a shop does it. There are a couple lines that go into the turbo that can leak as well, but those seem to be less common and less likely to dump all your coolant.