Try a solvent. It does look like glue/silicone. It's unlikely to be a filled crack, that would be some shithousery.
If I run a finger nail over it it definitely feels like a crack
Not good. I had some deep scratches on a sheet of glass that was installed, fortunately they did replace it after I pointed it out, hopefully you'll get the same resolution. Pain though.
@gillies started
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Try a solvent. It does look like glue/silicone. It's unlikely to be a filled crack, that would be some shithousery.