Radiator Fluid everywhere, no apparent leak anywhere?.?.?!

OK, I was driving on the highway this morning on my 150 mile commute to work. About 50 miles down the highway, I noticed in my side mirror there were plumes of smoke billowing out behind my car. I then glanced at the thermo gauge and it was pegged on H, overheating. I pulled over and popped the hood.

There was radiator fluid all over the engine creating said smoke. I whipped out my phone and turned on the GPS and noticed I was about 7-ish miles from a busy looking area. I had to go up about 3 miles on the highway, then venture a few miles off the highway.

After about an hour of sitting still, I popped the cap on the radiator and filled it with fluid again, and set off to find a shop. I expected that by the time I reached the 3 miles on the freeway, everything would be getting hot and smoking again. Well, it didn't. I watched my thermal gauge the entire time, and made it 100 miles with no issues. Got off the highway and drove a few miles in stop and go traffic to get to work, and the temp gauge did not go above 25% the entire way... AS IF NOTHING HAD EVER HAPPENED.

Now, there was obviously some leakage SOMEWHERE. Radiator fluid was all over my engine and the underside of my hood. I don't understand how I was able to fill it back up and drive another 100 miles at 70-80 Mph without it happening again.

Does anyone have any suggestions? It is obvious I need to have this looked at, but I do fancy myself as a very capable person when I know what needs to be replaced. Problem with this scenario is I have no idea where to even begin troubleshooting. There once was a leak, but now it is gone, as if the hole mended itself or something. Very strange.

What year/model/engine/miles? Could be your thermostat getting stuck. Depending on which model you have, it may be fairly easy to replace.

if it was low of coolant and overheated it very well could have pushed what was left out the overflow. i would start with the basics clean off the engine and top of the cooling system then pressure test it.

it's a 2002 Nissan Sentra/1.8L/135K miles

I actually drove all the way home to Atlanta yesterday after work (about 150 miles), then back again this morning and the temp gauge didn't move beyond about 30%, like it should normally sit.

I just topped it off with fluids about a week ago so I know it wasn't low on fluid. The thermostat is relatively new, about 4-6 months old.

What puzzles me is WHERE DID THE FREAKING FLUID COME FROM?!?! The radiator cap was securely tightened, the overflow reservoir was capped shut. I thought initially that I had busted a hose or something. After it cooled off, I refilled it and decided to drive a few miles up the highway and find a mechanic. I expected the newly added fluid would soon exit through the same "hole" once it began to heat up again. Not a drop...

I am blown away by this mystery. No leaks, nothing, yet there was radiator fluid all over my engine. Has anyone ever seen such a thing?

I guess I should add this...

A few weeks ago I did begin to run hot. I bought a gallon of antifreeze and filled it up. It appeared to be bone dry. Despite the actual radiator being empty, the overflow reservoir was filled up to the "MAX" line. I don't know all the details on how the cooling system works, but i imagine there is some sort of blockage keeping the fluid in the overflow from actually circulating to the radiator. Seems like it doesn't affect much, I just keep an eye on the temp to ensure there is fluid circulating.

Ditto on the pressure check. Could be a hole high up in the radiator, with fan spraying fluid everywhere. Some of the parts stores (autozone) will "sell" you a pressure tester, then buy it back when you return it. Also test the cap itself. The cooling system is supposed to run at ~ 7-10 psi; if pressure gets over 15 psi it allows it to pass into reservior. Ideally, any bubbles/gas pass into reservior first. When engine cools down, the coolant contracts and the system can suck any liquid from the bottom of the reservior back in. In this manner, over several days, the air bubbles get pushed out and only liquid coolant is in the system. That's why its important to always have some in the reservior.
Hope this helps...

i think it might be a bubble in the system somewhere the fluid could have came from the overflow tank from it getting to much pressure. the reason you could drive another 100 miles is because you were going highway speeds. wich means there was more air going into the enigine and less coolant trying to cool the engine down. it could also be the upper radiator hose. either restricted or loose or something. maybe a tear somewhere in the lines?