Had a customer who wanted to do a mailbox restore from 2 years ago but wasn’t sure which mailbox database the mailbox would have resided in back then. I found a really useful command to show the move history on the a mailbox:
(Get-MailboxStatistics <USER> -IncludeMoveHistory).MoveHistory | select CompletionTimestamp,SourceDatabase,TargetDatabase
Which gives you something like the below:
Hope it helps!