Slight YAW in Quad in All Modes

There is a slight clockwise YAW in an F450 that my 6 year old built. It occurs in STABILIZE, ALT HOLD, POS HOLD, and AUTO. As the same problem occurs in modes that do not invoke GPS or COMPASS data, I suspect Flight Controller is responsible, but I am not sure how to proceed with correcting the problem.

Check that there’s no trims set in the transmitter.
Check the motors are all straight and true, if one is slightly twisted that can cause it.

I’d check level horizon too, but that might not affect yaw. It seems most people level the flight controller, or just put the craft on a roughly level surgace - that might be OK if all the landing gear is perfect.
I use a lightweight level across the tops of the motors, and wedges under the legs to level the craft.

I have a frame with aluminium arms that have seen a lot of action (crashes) and can no longer be straightened and aligned perfectly, it takes a lot of time and eye-balling to get to fly without yaw if we have to pull it apart for some reason.

Log shows magnetic field issues too, so you might have some wiring work to do, or reposition the battery.
Graph of Yaw and Desired yaw, plus messages

Maybe make sure you’re voltage and current is accurately calibrated, then run the compass-mot procedure

Tracking of Yaw and Desired Yaw look pretty good. I am going to investigate Yaw Trim settings. A tiny bit of Yaw Trim on the RC Transmitter corrects the problem, so I am going to use Trim Save to transfer the settings.

Exactly what i was going to recommend. I do a Save Trim at the beginning of every maiden flight. Works like a charm.

Problem Solved. The problem was not that I needed a Yaw (RC4) trim but that I needed to remove a Yaw trim. During a drone event someone had swapped RC transmitters with my 6 year old daughter. She bound the new transmitter and completed RC calibration, somehow conveying a Yaw trim on RC4. I reset her RC transmitter to “factory settings,” rebuilt her 6 position switch and failsafe switch settings, completed RC calibration, verified the RC4 trim was gone (resent to PWM 1500), and flight tested. Yaw was gone!