I really don't know why you would ever want to remove such a mod -- while not rare as some other modded devices they are both becoming less in number/potential and desirable to the point where if it is a Jasper or anything newer then you will have a line of people willing to offer a trade or could alternatively sell it for more than a basic slim might run you if you are keeping controllers.
Anyway signs as in could someone with physical forensics detect it. Yes. Plastics, labels, solder type/style, possibly wear marks from wires/tape... and if someone did something like a fan mod then all bets off again. Main concern here would be trying for some kind of warranty replacement (don't know if any are still active, much less for a machine that might be modded).
Signs as in could someone with digital forensics detect it. Even assuming it was kept offline for the duration (so no possibility of a flag you don't yet know about), you wiped profiles (I know we discussed that in the past,
https://gbatemp.net/threads/using-an-xbox-live-profile-on-an-rgh-console.610463/ , but playing crazy safe in this scenario, wiped hard drives (or even grabbed one from another machine that was never touched) and restored a pre mod NAND backup. Possibly but it would likely be beyond whatever MS goes in for with bans in this day and age -- depending upon the wear levelling of the NAND sectors there could be some hidden things, possibly even able to be grabbed in software.