If it was overheating wouldn't the temp gauge go up? I always felt like maybe it was evaporating through the top tube.
Well yes and no. The temp gauges on these and most other modern cars are not really gauges at all; they are really just fancy warning lights. The gauge is center-weighted so that any temp in a certain range of "normal" operating temps will always show the needle in the middle. That "normal" range is very wide (50 degrees F is not uncommon) so if the gauge goes much beyond the center line, you are already headed for trouble and on the road to serious overheating. The time between the gauge rising past center and full-on overheating can be mere minutes in some cars.
The internal construction of the coolant reservoir is such that coolant would only come out the little rubber tube in an overheating or overfilled situation, so I very much doubt you're losing anything there unless the engine's actually overheating and forcing out the coolant.
Cars only use the reservoir to gain or release fluid during normal operation as the reservoir is not part of the active flow path for the coolant (nothing's pumping to/from there like the radiator). The coolant reservoir fills or empties based on pressure changes in the radiator and coolant jacket. Since pressure raises the boiling point in coolant (and any fluid), it is desirable and designed into the system. If the coolant boils and the pressure gets too high, it flows into the reservoir. When the engine cools off, the resulting vacuum sucks coolant out of the reservoir back into the radiator and engine.
BTW, The reservoirs so common today used to be aftermarket add-ons decades ago. I can remember people like my dad routinely adding them to cars to prevent overheating since back then the only way for the radiator to relieve pressure was by blowing the safety valve in the radiator cap. Once people added the extra reservoir (and check valve in the new cap), the overheating stopped as there was now an extra margin of capacity for the system when things got hot (happened a lot with early A/C systems overloading engine cooling) that didn't cause the system to boil off coolant and never get it back.
It is normal to have the coolant reservoir level go up and down between the High and Low marks during normal use. In fact, after you do a full coolant replacement, you usually need to add a little bit of fluid every day for the first few days as the air works it's way out of the system even if you follow the manual's bleed instructions. Check the level when cold before starting the engine. When hot, levels will likely be higher due to thermal expansion.
On the other hand, if the levels are dropping and not coming back up, then you are truly losing coolant. It's either burning (head gasket... white smoke), boiling (water pump, steam comes out when you open oil filler cap, chocolate milk oil) or leaking (real leak or being blown out the overflow).
I would check the cap on the reservoir to be sure it's holding pressure as a bad one can lead to coolant being blown out the overflow tube.
Assuming no leaks or other issues, you also need to figure out why the engine is forcing so much coolant into the reservoir in the first place since that only happens when overheating begins. Bad pumps, thermostats, clogged radiators, and faulty cooling fans are the suspects there.