Low coolant light comes on in 1996 Volvo when coolant level is OK

low coolant light comes on in 1996 Volvo. This used to happen, months ago, and I refilled the coolant tank and it didn't come on. Now it had started coming on periodically again, at the same times as it had months ago, like when I started up the car when the engine was cold, or when I pulled out at an intersection when the light turned green. Then it came on & stayed on, yesterday. I checked the tank and it was pretty empty. All I had available in the car at that time was a bottle of water (no antifreeze), so I filled up the tank with water, up to the max line. The light stopped coming on, for a day. Then it started coming on again today, so I checked the tank and some water had been lost from the tank. So this time I bought a premixed antifreeze+water bottle and filled it up to the max line. The light unfortunately keeps coming on, staying on most of the time now. What is causing this? I don't think the sensor is completely broken since it has several times correctly indicated an actual loss of fluid in the tank. But why is it staying on now that the tank is full? What would cause that? It's a bit unnerving to look at the dashboard and constantly see that warning light.

I fully realize that there is probably a leak somewhere, which I know is quite a problem! But I can't afford repairs (or a new car), so all I can do now is try to nurse this old car along. I'm guessing the best thing I can do (without $ to get it fixed) is just to keep checking the tank level every day and adding more antifreeze + water as needed, to keep it up to the max line - is that right? Or is there something else I should be doing?