RADIATOR LIGHT ON DASH

I have a volvo v 70 1999 edition. I did get the car used and the car works very well. Recently I noticed that I was using alot of coolant and when it got low the light on the dash came on and I filled the reserve back up and the light when off. I bought so stop light stuff to put in the collant reserve. About a week later the light came on again so I went to go check the reserve to see if it was low and it was still as full as it was over a week ago when I put the stop leak in. The car isnt over heating and is still driving as normal. Do I need to get the the radiator flushed to get it out of the system or do I need to have the sensors checked? And if its the sensor where are they located?