Your logic is correct, the address-list name will appear as a drop-down in Winbox if it is mentioned *anywhere* in config. When you exported, did you export the entire config, or just the firewall section? If the address list contains anything other than alpha numeric characters (like a hyphen), it will be inside double quotes in the export.
Hopefully you can export the entire config and find it that way. There is no other way to remove it from the available choices than to remove all references to it in the configuration. Since Mikrotik does not have the concept of running-config versus startup-config like C!$c0, it can't be hiding anywhere else (there is safe-mode, but that's just not the same, nor applicable in this inquiry).
As a reference, I just exported a verbose config and find address-list variable used in the following sections:
/ip firewall address-list (duh!)
/ip firewall filter
/ip firewall nat
Note that I do NOT have all available packages installed on this router, so there may be more instances hiding in the config of extra add-on packages.
- If I helped you solve your problem... I am now able to accept tax-deductible Karma donations!