This often indicates a failed heater core. You can get another indication of a failed heater core when you use your defroster. If you find an oily residue on the windshield when you use your defroster, this could indicate a failed heater core.
The heater core is a component inside your HVAC assembly that warm coolant runs through to provide the passenger compartment with heat. It’s like a tiny radiator, and, like a tiny radiator, it can occasionally leak. Repair of this often requires removal of the dashboard and all of its components to access the HVAC unit. It can be involved and time consuming, and if you have a shop do it for you, it can be very expensive.
That said, there is a workaround. You can often bypass the heater core by rerouting the heater hoses in the engine compartment. On the firewall or bulkhead, you’ll see two coolant hoses going into and out of the heater core. If you disconnect these lines and either route them together or remove one and route it back to the engine, you have in effect bypassed the heater core. Now, doing this means you won’t have any heat, but you won’t have a leak anymore either. You be the judge in your situation of whether this is a good idea or not. It will not harm the engine or cooling system if you do this. The only reason you have a heater core is for your comfort, not the vehicle’s.
Once again, using stop leak products is not recommended. If you find you have a coolant leak, I strongly recommend you repair the leak rather than use a stop leak product. Yes, there are times those products work, but they come with a risk. If you perform the repair instead, the risk is greatly reduced if not eliminated altogether.