I had an issue when I removed kernel 5.13 via Manjaro Settings GUI and manual intervention was required to remove files from /boot. I found a similar topic that suggested the same action. If uncertain, make a backup of the files first. I suspect that grub makes no reference to these non-listed kernels.
A side note: I use locate -i (alias loc) to find things (see updatedb & systemctl status updatedb), at least to narrow it down, and then use find.