Mobile Security seems to think there's an unverifiable app on my phone?

In the last few weeks, Mobile Security on my Pixel 3a has been regularly throwing up overnight a notification that there is a new app installed that cannot be verified and that a scan is required.

The weird thing is, no new app has been installed. Of course I worried that there was some malware that had made its way in, but the scan I am now doing every day always says the phone is clean.

I've discovered that there's no way in Mobile Security to work out what the app is that Bitdefender objects to. That really should be part of the error message. I have no idea where the problem lies or what app is causing the notifications. After many scans I am reasonably sure the phone has not been infected.

Any ideas?