Windows Powershell ISE - "Global" variable $SearchFolder causing havoc

Question

Disclaimer: I am pretty new to scripting and my terminology might be off, but I will describe the issue as good as I can.

I executed a script with an undeclared variable $SearchFolder in Windows PowerShell ISE. Here is what happened.

Script's logic: Define two folder variables, say A and B, and move content of A to B

Version 1:

$SearchFolder = "E:\folderA"

$ProcessedFolder = "E:\folderB"

Get-ChildItem $SearchFolder |

Where-Object { $_.Attributes -ne "Directory"} |

ForEach-Object {

#Move-Item -Path $_.FullName -Destination $ProcessedFolder

}

Worked exactly like intended up to here.

I then rearranged my variable naming convention ('Folder' as pre- instead of post-fix). I unfortunately missed out on renaming the variable in the Get-ChildItem line, though, i.e. script looked like:

$FolderSearch = "E:\folderA"

$FolderProcessed = "E:\folderB"

Get-ChildItem $SearchFolder |

Where-Object { $_.Attributes -ne "Directory"} |

ForEach-Object {

#Move-Item -Path $_.FullName -Destination $FolderProcessed

}

In other words: I declared variable $FolderSearch never using, but accidently called (un-declared!?) variable $SearchFolder instead.

I assumed that a script will fail if it encounters an un-declared variable. But it executed and started moving system files and folders from my C: drive (Program Files, Program Files (x86), Users,...) to FolderProcessed! I killed the script pretty quickly, so not entirely sure what it would fetch if left undisturbed. So even though undeclared, $SearchFolder seems to assume some "globally assigned" values.

Anyone who could shed some light on this?