"Burn" looking marks on material

[rydods]

There's a sign we put up last year and it looks as though the material or laminate is darkening in a couple of spots (photos attached). Any idea what might be causing this or how we can avoid it. It seems to be only at the top of the sign. Material used, Lumina 7300 reflective with Avery 1360 gloss laminate.
Quick note, the posts were stained before we installed the sign and it's at the edge of a farm field in the middle of nowhere so I kind of doubt vandalism would be the issue.

 

[Solventinkjet]

The materials you used seem to be fit for the job. If you're printing solvent ink, how long did you let it off gas before laminating? If not long enough, it can cause issues like this.