The first thing I would do would be to count the cards in the deck, hand, and battlefield to verify the amount. I would pull the deck list to verify the Overgrown Tomb belongs in the deck before taking the player's word it does. I would also count and verify the sideboard quickly to make sure the card was not in there and the player forgot to de-sideboard.
I agree with Callum about the resolution of issue and penalty. If all checks out, I would issue the player a Deck Problem Warning penalty. I would not upgrade due to the player calling it as soon as he noticed his deck had an issue and being early in the match. If the investigation findings are he discovered it under his chair, it was an issue with this round and not previous rounds as most players move locations between rounds.
Edited CoriAnn Theroux (March 19, 2018 10:39:38 AM)