Fick pengar till bankkonto men vet inte varifrån?? Banken är ohjälpsam

Hej allihoppa, jag bor i Sverige sedan några år men min svenska är inte 100% så ja ska ta detta på engelska.

At the end of November, I received 7200 kr into my Nordea personkonto. However, I did not send it there myself and neither did my family, friends, partner, job nor university. 7200 kr is also a fair amount of money, so I would remember transferring that much to myself. Unhelpfully, all I can see in my list of transactions is "insättning" followed by a 9-digit number. Upon calling Nordea, they told me that they can't see anything apart from the money originated from a Handelsbanken account and that if I would like more information, I have to pay 60 kr for them to do an utredning to find out the account number it's from.

I find this to be a very strange proposal given that I'm being a 'good citizen' by declaring unexpected funds arriving in my account and yet I am very strongly disincentivised from actually doing anything about it. I think it's quite silly for me to pay to find out an account number.

How should I proceed? Does anyone have any tips for what I can do?

EDIT: thanks everyone for the tips! I'll reach out to Nordea in writing and discuss options with them. Your help and replies are much appreciated!