Below powershell helps you the get the list of the entire Hidden mailbox in the Exchange Organization

Get-Mailbox -ResultSize unlimited |Where{$_.HiddenFromAddressLissEnabled -eq $true}

If you wanted to the get the list of Hidden maibox from the specific Exchange server then

Get-Mailbox -ResultSize unlimited |Where{($_.servername -eq “<servername>”) -and ($_.HiddenFromAddressListsEnabled -eq $true)}

If you get the list of Hidden Mailboxes from the specific mailbox store then please use below command

Get-Mailbox -ResultSize unlimited |Where{($_.Database -eq “Server\SG\Store”) -and ($_.HiddenFromAddressListsEnabled -eq $true)}

Powershell to command to get the list of Hidden DL from the Orgnization

Get-DistributionGroup -resultsize unlimited| ?{$_.HiddenFromAddressLissEnabled -eq $true}