Test-SPContentDatabase - missing feature

A missing feature in content db refers to orphan references to uninstalled features.
Painful among these would be the features that are scoped at web.

One possible cause of this could be the way the feature was uninstalled. if the feature was uninstalled with powershell script logic
   $subwebs = get-spweb -site xxxxx
   foreach($web in $subwebs)
            disable-spfeature -identify yyyy -url $web.url

it will appear fine but fail in some farms where the number of sub webs in high use
 $subwebs = get-spweb -site xxxxx -Limit all

is the right way


but this is postmortem, what do we do about features that are missing at web levle

There are quite a few things you may have to do
1. Identify the missing feature
              you can do that by taking the feature id from test-spcontentdatabase command and searching it in dev environment or your source code repository

2. Re-install the missing feature
               bring it back to the environment - reinstall it

3. Deactivate properly at all web levels
                write powershell script to loop through and disable the feature at all levels

here is a sample code:

Add-PSSnapin Microsoft.SharePoint.PowerShell -EA 0
$featureid = $args[0]
$featuretocheck = Get-SPFeature -Identity $featureid
Write-Host "Checking for feature" $featuretocheck.DisplayName
$webapps = Get-SPWebApplication
foreach ($webapp in $webapps)
{
 Write-Host "looping through webapp" $webapp.url
 $sites = Get-SPSite -WebApplication $webapp -Limit ALL
 foreach ($site in $sites)
 {
  Write-Host "looping through site" $site.url
  $subwebs = Get-SPWeb -Limit ALL -Site $site
  foreach ($subweb in $subwebs)
  {
   Write-Host "verifying web" $subweb.title
   $feature = Get-SPFeature -Identity $featureid -Web $subweb -ErrorAction SilentlyContinue
   if ($feature)
   {
    Write-Host "Activated -" $subweb.title -ForegroundColor Red
    #Disable-SPFeature -Identity $featureid -Url $web.url   }
   else
   {
    Write-Host "Not Activated -" $subweb.title -ForegroundColor Green
   }
  
  }
 }
}