Beat the game and looking for secrets!

Any secrets or eater eggs found? .. Extra bosses? HOW TO OPEN THAT DANG DOOR RIGHT BEFORE THE BRIDGE?!?!

Edit1 WARNING:stock up on money and all items before doing this!* True Lab: After getting both Undyne and Alphys as friends ( giving going to Undynes house and Giving Alphys the letter) you will get a phone call telling you to go to Alphys lab and see her.. by the bathroom there is a note. The Bathroom can now me entered for a new area to explore. I'm not sure if you have to beat the game first in order to go to this area. EDIT1 Special note! if you do this after becoming friends with Undyne and Alhys this will lead to a new ending! No Details for you! just do it! BE PREPARED and ....stay determined