Compare two indexes using Sitecore PowerShell Extensions to see what's missing.

CompareTwoIndexes.ps1

# This is a ScopeQuery item found in SXA. You don't have to use this.
$scopeItem = Get-Item -Path "master:" -ID "{648F4C3A-C9EA-4FCF-82A3-39ED2AC90A06}"
# If not using SXA, you can replace this with the string from the QueryBuilder field type.
$scopeQuery = $scopeItem.ScopeQuery
$props = @{
    Index = "sitecore_sxa_master_index"
    ScopeQuery = $scopeQuery
}

$referenceList = Find-Item @props
$referenceLookup = @{}
foreach($entry in $referenceList) {
    $referenceLookup[$entry.ItemId.ToString()] = $entry
}

$props = @{
    Index = "sitecore_sxa_web_index"
    ScopeQuery = $scopeQuery
}

$differenceList = Find-Item @props

$referenceIds = [System.Collections.Generic.List[string]]@()
$referenceList | ForEach-Object { $referenceIds.Add($_.ItemId) > $null }
$differenceIds = [System.Collections.Generic.List[string]]@()
$differenceList | ForEach-Object { $differenceIds.Add($_.ItemId) > $null }

$referenceHash = New-Object 'System.Collections.Generic.HashSet[String]'
$referenceHash.UnionWith($referenceIds)
$differenceHash = New-Object 'System.Collections.Generic.HashSet[String]'
$differenceHash.UnionWith($differenceIds)
$leftOnlyHash = New-Object 'System.Collections.Generic.HashSet[String]'($referenceHash)
$leftOnlyHash.ExceptWith($differenceHash)
$missing = $leftOnlyHash | ForEach-Object { $referenceLookup[$_] }
$missing | Select-Object -Property Path | Sort-Object -Property Path | Show-ListView