If the 17th tower is destroyed then the 1st returns.
As an example, if you had a 4 vs 4 game where everyone was Terran, each player could only make 2 sensor towers before the circles start getting removed.
Although towers are fairly expensive (125 minerals, 100 gas), they could potentially be exploited (e.g. An opponent attempts to fly Mutalisks into your mineral line not realising that its within the sensor tower radius).
This bug could be by design however, to prevent the minimap from getting too cluttered.