Coolant Leaks without Overheating Signs of Overheating

I'm new here and I created this account specifically for this question. I drive a 2013 automatic, and am currently driving cross country and about 500 miles ago the maintenance required light turned on. Buddy and I checked it out at a gas station and the coolant reservoir was at about 1/5th the level it should have been.
Today I checked the reservoir and it had drained back down to the same level it was before we originally filled it.

The temperature gauge was in the middle, without being close to overheating. There is a chance that gauge might be broken since I have barely seen it move, but I am worried about jumping to conclusions as I am a new Tacoma owner and still relatively new to driving (19 y/o)

I will check the reservoir in the morning because my buddy suspects that the reservoir may have a crack in it, causing it to leak to just below the leak and no more, leaving enough to not overheat.

 

The only thing the maintenance req'd light comes on for is your oil change reminder. As far as the coolant level goes The reservoir is only holding extra coolant for variations in pressure and vacuum situations in the radiator temps. Crawl under there and see if you have any dried white or wet pink stains/puddles anywhere. That will tell you if it is leaking. As far as the gauge, right below half is operating temp. If when you start it and it reads cold and reaches right below half then your gauge is working.

 

The Maint Required light is an oil change reminder, it blinks for a few seconds at key on 4500mi after it is reset and stays on steady 5000mi after it is reset.

The overflow bottle is just to allow for expansion of the coolant in the radiator so you really want to check the radiator as well when the engine is cold and keep the bottle between the 2 marks, they have been known to crack and also the plug at the bottom is known to leak so check that first.

 

I went out and checked, as far as I could tell nothing below the engine leaking out or as stains. I will check again in the morning to make double sure.

The temp gauge is working then, thank you.

I'll check the bottom plug when I check tomorrow morning.

 

If it is cold right now open up the radiator cap and fill it up to the brim if it is not already.

 

In the morning, it'll be good and cold, a perfect time to pop the radiator cap off. So instead of pressure in the radiator, there might be a slight vacuum, coolant probably won't spill out. If there's coolant still in the reservoir, the radiator should be full to the very top, or within a inch. If the reservoir gets empty and you have no coolant to add, plain water is better than nothing.

I wouldn't worry given your situation. Just glance over at the temperature needle occasionally, don't use it as a place for your used sunflower seed shells or anything silly like that to obscure your view. Just drive, enjoy the scenery, snap us some pics along the way.