ETA

Hi, my girlfriend and I arrived to Edinburgh few days ago. Nobody at the airport checked if we had ETA's, I scanned my passport and went through but my girlfriend couldn't because she didn't have ETA (me neither but I passed without any problem). After some talking the airport guy let her pass as well but told us to buy the ETA's. We are leaving tomorrow and my question is; Do we need ETA's to leave the Edinburgh or nobody will check it?