calpurtmun from my understanding and if you only used GOS on the phone and flashed Pixel OS back for trade-in, nothing really can happen. The storage is wiped and your data inaccessible (not sure what could be done with lots of efforts to devices before the Titan chip era, but who would try and why?).
Google could check on hardware identifiers and figure that at one point in time someone connected the device to the internet (maybe they still have the IP from back then) and activated OEM unlocking. If that was not your IP (e.g. public hotspot) it's not even reasonable to assume that the same person doing the trade-in was originally buying it to flash another OS on it.
So nothing to worry about, but let's wait for some real expert to confirm.