Yesterday during GP Patis main event day one, while I was lurking at the bottom of the standings (I am agreat standard player!) I got involved in a case where I think I should jave done more. Here is the case :
We are at the end of the round. After the 5 additionnal turns I lost my fourth round in a row(so I end with 0-4 drop). The match end with a 0-1-1 due to IMO a slow opponent pace from a low magic experience (personal oppinion). While discussing a little bit with my opponent we arrived at tje point wjere he tells me he has a few foil cards in his deck. I ask him if I can check his deck and he give it to me. After seceral cuts I reveal a foil card in top deck position. I then ask him what could be an issue and strongly advise him to get to the HJ.

Hence two questions :
-Should I directly had to involve a judge(we are in the end of round procedure)? I now think I should have.
-If a marked cards error is discovered between rounds, can a penalty be issued or is it working the same way as Deck(not decklist) problems?

I would always tell a judge about a marked card (cheating in general..), i mean if it wasn't on purpose then ye, nothing happens. (The judge can explain to the “unexperianced” player why he should avoid building a deck where this can happen..)
If it was on purpose, then cheating was involved and we got one less cheater.

In short Yes.

Remember, the point of giving warnings for players is that they are tracked. We cant be expected to know every player we see at an event and their track recorded. This could be his first error even, or the 4th event in a row with a Marked Card warning. Sometimes a player doesn't know that something is an issue until you lay it out for them.

I've deleted the responses due to some issues with the direction this discussion was going.

While Olivier does not mention that the cards in question were marked, just that they were foils, it seems obvious to me the issue was with the specific curvature of those cards, compared to the rest of the deck, not that they were specifically foil.

When we as judges are playing, it can be hard to switch off our judging instincts and desire to report all possible issues, as players the expectation of our responsibilities shouldn't be more than those of other players. With that said though, our understanding of why policies are in place and why reporting things to judges are a good thing shouldn't be ignored.

It should never be considered wrong to call a judge if there appears to be an infraction being committed. It also is not a requirement to call a judge on an infraction witnessed in another match. Where this gets a little tricky is you are required to point out rules or policy infractions in your match, does this also mean after you've completed your match and are just discussing things? I personally think it does, so if you notice your opponent has marked cards after the match is complete you should report it, as Jarrett mentions, tracking these things can be useful.