coolant constanly low, no external leak apparent

If you are getting any kind of sweet smell from the exhaust it could be a bad head gasket. I would start by pressure testing the system to see if it holds pressure and to double check there is no leak. If it isn't holding pressure then you can buy a head gasket checking tool that is able to measure exhaust gasses in the coolant bottle or radiator. If you don't work on your own car then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex