After 16 months, all your rock and sand are now "live". As thincat mentioned, it just means it is populated with bacteria that convert ammonia to nitrite, and nitrite to nitrate.
Which also answers the question of why you don't see any nitrites - you obviously have enough bacteria to convert the nitrites to nitrates quick enough that you can't see any.
Regarding your nitrates... what kind of water are you using for your water changes and top offs? Tap water? RO
water? Have you tested your newly made salt water for nitrates? If you're using tap water, the nitrates could be coming in with every water change.
When you say "high" nitrates, what kind of levels are we talking about? 5 ppm
? 20 ppm
? 100 ppm