Test-Connection -Quiet -Count 1 Hostname
gives output

Test-Connection: Testing connection to computer 'Hostname' failed: Cannot resolve the target name.

Expected behaviour would be

False

In a code block like
$comps | ForEach-Object -Process { if (Test-Connection $_.Name -Quiet -Count 1) {Write-Output "$($_.Name) on"} else {Write-Output "$($_.Name) off"} }
this gives output
Test-Connection: Testing connection to computer 'Hostname' failed: Cannot resolve the target name. Hostname off

Again, the extra output is unwanted, given the Quiet flag.