Potential Virus on Grandma's Computer – Scans Show Nothing

My grandmother's computer keeps getting these pop-up ads claiming there are viruses and other urgent issues. I've seen the three pictured, plus a few other variations with fake Windows or McAfee logos. They show up randomly and she's worried she's infected.

I restarted into safe mode, ran the internal scans from Windows Security (quick scan and the offline deeper scan), and also did a full Malwarebytes scan. Everything came back clean—no threats found.

But the pop-ups just won't go away. I'm thinking it might be a browser notification scam or some adware that's slipping through, but I'm not entirely sure. Anyone dealt with something similar or know what else I should be checking?