Likely your problem is the temp sensor O-ring in the lower radiator hose. Very common problem, even if it is not leak, it needs to be replaced. No tools needed, very little coolant lost as system vacuum keeps coolant from leaking.
Technically the O-ring is not a serviceable part.
See my signature for the O-ring part number.
In a pinch, you can pull sensor, remove O-ring, 4-5 wraps of teflon tape in O-ring groove, reinstall.
If the switch is wet at all or if there is any wetness on the lower engine cover it is leaking.
Other than that, expansion tank is a likely suspect.