There's like 3 of them though. I completed the lab level. The one at the waiting room told me to go back and said there's something else, but I can't find it.
Oh, now I get what you mean.
She means you can go and complete the other stages of Forbidden Lab (both Big PP and Furry stages).
There's a certain item in each stage that you can use at a secret area of Forbidden Lab to unlock some lore material.
Also, I just loaded back in and now both the starter lady and the no battery bot are there again as if I never completed the level... but my clear time is still there...
This happens at any stage so you can replay it from start. The only difference is Forbidden Lab has a secret area that unlocks by collecting one of the three secret items at the lab (one at each stage). Also, you can use the exit from that secret area to skip the boss.
Edit: Note that you need to go through any of the three portals to reach the boss at the lab. That means you can go through the same portal three times, but the stage will become harder the more you use it.