Had been dm-ing for awhile, then had a long break do to covid. The monk has a +15 to hit on unarmed strikes. 

My math gets him to +12, (prof + dex +1 for the insignia of claws) 

 Can anyone else figure it out? 

 https://www.dndbeyond.com/profile/Odenathus/characters/12270755

Thanks!

I noticed both the other weapons are customized. Have you tried unequipping them to see if that affects his unarmed strikes? The +3 from the staff might be getting added somehow.

I took a quick look and did not see how it happed. It is supposed to be Dex+Prof+magic+other, you know the first 3 are 5+6+1=12 so some how there is extra listed. As a test you can also simply recreate the PC and see if you get the same numbers incase there is a error else where that is generating the result. ie do all of the standard stuff first then add in the custom stuff one by one and see where there error is.

I would do you math, show it to the player and see what they have to say and work out something from there. Try and do it before the game, ie do not blind side them right before the game so they have time to explain what did or might have happened.

Good Luck

Well, I compared it to my level 20 monk, and added the Staff of striking and the Insignia of claws.  Observations:

1. The attack bonus for his Staff is +12, on mine it is +14.  The damage bonus is the same in both cases.  So either it customized to lower the attack bonus, or its a homebrew that is misconfigured.  
2. The Unarmed Strike is +12 on mine, and +15 on his.  But his Unarmed Strike isn't customized.  I tried customizing mine, and it clearly shows customized.
3. I couldn't look at the homebrew "Mace of Terror Exalted"   But I created my own.  In it I just added the proficiency (so my monk could equip it) and set a modifier "Bonus - Melee Attacks" with a fixed value of 3.   The result was the same as what you see on his character sheet.  Here is a link to mine so you can see the results: Copy of Ss'Thak for the forums . 
4. Since my Monk is a Kensai, it also added that bonus to my Vorpal sword AND to the Staff of Striking.  This means that on the player's sheet there is a -5 either on a Homebrew Staff of striking, or -5 to hit on the customization.

So, my best guess is that the customized "Mace of Terror Exalted" is using the wrong bonus type.  If it was intended to make it a +3 mace, it should be using the modifier "Bonus - Magic" with a fixed Value of 3.  But the customized text doesn't indicate it has such a bonus.

 I hope that helps.

This is exactly it. Thank you, and thanks to everyone that took a look!