Finding and removing Android adware that opens Chrome tabs when unlocking the lockscreen
Using my Google Activity History, i spotted a few activity entries of my Barcode Scanner app which didn't belong there as I didn't use the app at the time.
Uninstalling the app resolved the problem. In my case, the com.qrcodescanner.barcodescanner] was the culprit.
Edit: The app has been pulled from the store since.
The most recent 1-star reviews confirm that the apps is an adware since the last update.
For completeness' sake, this might not be the dev's fault since the app Andrew pointed out (com.google.zxing.client.android (Play store)) suffers from the very same condition it seems, the reviews now all say it delivers ads.
Edit: Apparently this is a false positive, as it unfortunately had an identical app name and got hit with negative reviews for no reason
Edit: Maybe some barcode library further upstream got compromised?
In any case, caution on using either of the apps is advised.
Use zxing's Barcode Scanner for a better alternative
Credit to @andrewT. for pointing me to the solution
Update, 08. Feb 2021: There is now a writeup of the story at Malwarebytes Blog