I tried this yesterday after turning off the firewall, but nothing appears in the list... Only a green bar appears with no text.
The firewall blocks a lot of traffic coming into the NAS but I assume turning it off should have given some results? I can't see that there is no certificate, but I can't confirm that there is either...
I would perhaps expect a red bar or some information telling me that no certificate was found if that was the case, but I see nothing...
Again - this could be because of some firewall functions that have not been completely turned off...
Thank you SLDan. This sounds like way above my knowledge level... 
I guess I just have to trust and believe that this was a certificate renewal. I find it somewhat strange though that I cannot find a current certificate number on my NAS that I can check with the one displayed on my phone.