Doing, this...
Windows Explorer (File Explorer), right click on any file, choose
Properties > Security > Edit > Add > Advanced > Find Now
... is only showing user who have perms on the file. What does that have to do with Service Accounts? If you want the list of users / perms on a file or folder, that is what Get-ACL is for.
# Get parameters, examples, full and Online help for a cmdlet or function
(Get-Command -Name Get-Acl).Parameters
Get-help -Name Get-Acl -Examples
Get-help -Name Get-Acl -Full
Get-help -Name Get-Acl -Online
(Get-Acl D:\Temp\fsoVolume.docx | Select -ExpandProperty Access).IdentityReference
As for this...
Theoretically, can't there could be a service account which happens not to be used by any service?
... Nope.
What you are saying here is not valid. Simply put, a service account which is not assigned to a service, is simply a user account.
There is no way you are ever going to know if some user account is used as a service identity, unless you know the service you are looking for and know the account that should be assigned to service.
Again, A service account is only a service account when it is assigned to a service. No matter what it is named.
Most organizations have a taxonomy / naming convention for accounts which will be used for a given service. Usually, something like svcSQlService, or the like. Yet, if your org has no standard, then that is the first thing to address.
You can only list all services and see what account is assigned as it's identity. If your org has a naming construct, you can use that list to compare against all services to see if it is ever assigned to anything. If not then it should be either assigned to its target service or deleted as some random user account.