The user has appropriate permissions to read the share, and the share maps properly. This issue seems to happen only in PowerShell v2.0.
If I remove all mapped drive
If the Test-Path cmdlet is truly buggy, then I'd suggest using the Exists() method on the System.IO.Directory .NET class.
Test-Path
Exists()
System.IO.Directory
[System.IO.Directory]::Exists('G:\'); # This returns $true for me