I ejected one tape/media from NBU activity monitor and forgot to copy its mediaID and it got mixed with other tapes, Now I wanted which tape i ejected , how i can find out.
try following the same steps that you did to choose to select the media, if that does not help you need to look into the media manager logs
check the below excellent post from Marianne for details.
robots or daemon logs should have tape eject info.
There is no other place in NBU where eject info is logged.
If you have NOT manually created media manager logs (in volmgr/debug), then your only option is to re-trace your step.
You must've had a valid reason to eject a tape, right?
How do you normally select tapes to eject? Maybe tapes used for backups in a certain period?