Coolant overflow

First off, I want to say thank you for everyone who takes the time to answer these questions. Im not mechanically inclined and I'm learning as I go, so be gentle lol.

2015 freightliner cascadia DD15 with coolant over flow. Apologies if I'm not saying this correctly. I forgot to take the truck out of gear when parked and the truck jolted but I didn't fully release the clutch. The yellow check engine light came on that says "check." Drove one block and parked and noticed coolant in the overflow part of reservoir as pictured below. I tried checking the EGR sensor but didn't have the right size socket to fully remove it, but was able to remove the pipe(also pictured below, to check for coolant or water, but there was none in the pipe, which i assume that there world be none in the sensor, but in unable to get it off to check. Haven't checked the thermostat, and haven't checked the pressure in the reservoir or radiator. I checked the oil for water or coolant on the dipstick but found none. I was told by a mechanic that it wasn't over over heating (which it never went over 200°) that I was fine.

Drove 200 miles with no issue, and oil pressure is about 50 in highest gear, and 15 to 20 while idling. No smoke, or loss of power, and no oil or diesel in the reservoir. I don't know if matters but 2 weeks ago there was fuel in the reservoir, and replaced the piston rings, and cups and flushed the radiator and reservoir like 5 times, and the green coolant was all I could find, and have not mixed in the orange with it. The truck previously lost power going up hills and changed the fuel filter oil filter and air filter which seemed to fi the trick. Also, I did check similar forums and I don't know if it's a blown gasket or cracked head, but I assume that the lack of smoke, and/ or coolant mixed with the oil that I can rule out.

I tried being a thorough as possible with explaining and what we've done so far. The check engine doesn't come on, and the truck has absolutely no issues, but I don't want to take the chance and mess it up. I would appreciate any advice. Thank you!