You should try running a second or third on demand scanner (such as Malwarebytes Free and SuperAntispyware Free).
Comodo isn't 100% full-proof. Nothing is.
https://www.malwarebytes.org/
http://www.superantispyware.com/index.html
I've had luck with both. For me things that slip-past MAM Free have been caught by SAS Free.
Hence why I still use SAS Free.
As for the Mac, I'd get http://www.clamxav.com/ either from the official website there or the app-store (note the app-store version doesn't provide any option for file/folder monitoring so you only get the ability to run manual scans). Macs do get viruses. And there have been plenty of cases (hundreds and hundreds) outside of "testing labs" (real world infections) despite what lies I've seen get spread around forums (including here).
On linux I don't know if others do this but I know Ubuntu has ClamAV built-in, so I would go to the app store or whatever your distro calls it, and grab ClamTK, then before running it I would open a terminal and type "sudo freshclam" to make sure it's 100% up to date then I'd open ClamTK and run a scan.
This way you can assure that all of your systems are clean of infections.