Burn marks on outside of log burner glass

[namechange2222]

That's it really. I've had the burner for a couple of years and just generally give it a wipe once a week and the glass is fine. In the past few weeks I've noticed a burn which looks like it's stuck and getting bigger on the outside glass ( almost as if something has melted on it but pretty sure it hasn't)
Nothing I have is removing it and wondered if anyone has any ideas please

Wiping with a damp cloth dipped in some wood ash usually works for me. I'd want to know why it is happening though. Is the door seal (string) damaged? Is the glass seal OK?

Thanks Muster. Well I'd tried all the usual remedies including wood ash so today I've used a scouring pad with some warm water. It's come off and the glass looks OK
I've absolutely no idea what caused it, all seals are fine. Ive had burners for around ten years and never had burning on the outside!

Most likely something dripped or splashed on it.