This script will find the last logon date for a supplied user on each DC in your domain and output to a text file. You can then pull the text file say into Excel and sort on the date column to find when the user last logged in.
(You need the Quest AD cmdlets to run this one)
$DomainControllers = Get-QADComputer -computerrole domainController
foreach ($DC in $DomainControllers)
$a = Get-QADUser -Service $DC.name 'domain\user'
$dc.name +" " +$a.lastlogon | Out-File -Append C:\Scripts\LastLogon.txt