If the cars passed the pre-race inspection then this is on NASCAR. Can’t say “you’re good to go” and let them race and then say, “oh sorry we missed this.”
As part of the post race inspection, the wrap is pulled back to verify panels are unmodified. They can't do this pre-race because A. it would damage the wrap, and B. it would take FOREVER to do on every car. This was part of the normal top 5 post race tear down.
They could face a much higher penalty, an L3, because it was a modified single source part. The body panels are supplied by Five Star and cannot be modified in any way. That is why the cars can no longer be painted and can only be wrapped.
I am honestly curious if the other 2 Gibbs cars and the 2 23XI cars had the same issue, but were not torn down since they were not part of the post-race inspection. Gibbs could argue the components were supplied that way, but that would have to be part of their appeal, if they choose to file one. I am also curious if this was done at Gibbs after the parts were received from Five Star, how far up the chain of command was there knowledge?