What does "C:2\" in a windows path string mean?

I got this pop-up from the Windows firewall. What is "C:2\" in the path? The real path is D:\Steam\SteamApps\common\...

I tried cd /d C:2\ in cmd and got "The system cannot find the path specified."

I also tried cd C:2\ in Powershell and got "Set-Location : Cannot find path 'C:\2\' because it does not exist."

So how is "C:2\" a shortcut for "D:\"?

Update:

I tried searching for C:2 in the registry, as @Tyson suggested. There are a lot of irrelevant search results when I search for C:2 in the registry with "Match whole string only" unchecked and no results with "Match whole string only" checked. Yet I found the key, it's in HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\services\SharedAccess\Parameters\FirewallPolicy\FirewallRules and its value is v2.10|Action=Allow|Active=TRUE|Dir=In|Protocol=17|Profile=Private|App=C:2\steam\steamapps\common\sonic & all-stars racing transformed\asn_app_pcdx9_final.exe|Name=asn_app_pcdx9_final.exe|Desc=asn_app_pcdx9_final.exe|Defer=User|. There is a rule for D:\Steam\SteamApps\common\Sonic & All-Stars Racing Transformed\ASN_App_PcDx9_Final.exe in Windows Firewall with Advanced Security > Inbound Rules. I also found a rule for Team Fortress 2, which is v2.10|Action=Allow|Active=TRUE|Dir=In|Protocol=6|Profile=Private|App=C:0\steam\steamapps\common\team fortress 2\hl2.exe|Name=hl2.exe|Desc=hl2.exe|Defer=User| in the registry and D:\Steam\SteamApps\common\Team Fortress 2\hl2.exe in the Windows firewall rule list. Other games installed in D:\Steam\... have D:\ both in the registry and Windows firewall rules. So it's probably how Steam makes aliases for drives other than C:\, but I'd like to know how it works, and why in one case it was C:2\, in another C:0\, and in the third one just D:\. I guess it's probably because it's an external HDD, and sometimes Steam couldn't find it the moment Windows woke up after sleep, so Steam assigned different aliases for it.

But what actually makes those aliases work? There are no folders called 2 or 0, it's just D:\Steam\...

Solution 1:

#1 ADS

Could this be a file called C, which is actually a junction (symbolic link) accessible from the app's PATH, and for which there's an alternate NTFS stream ADS (Alternate Data Stream) so that C:1, C:2, C:3, etc. would resolve to a different location? Since Steam have been limited to single-drive installation for most of its existence, perhaps this was a hacky workaround to support relocating games?

Main issue: I'm not aware that ADS worked with folders, shortcut targets, links or junctions. However, if it did, that would bring it pretty close to this syntax.

#2 Relative path

On the link posted by KingZoingo, we can see that C: could be the current relative path:

"C:tempdir\tmp.txt" refers to a file in a subdirectory to the current directory on drive C.

Since it's legal to name a folder or junction "2", that would also lead to this syntax.

Main issue: I would have expected the Firewall to show the actual absolute path.