Muppet Babies - Season 5 - Eps 4: Is There a Muppet in the House?
Rowlf hadn't disapeared - he'd just watched a scary movie the night before and had gone to see Nanny. The secret passage was actually an old dumbwaiter which Animal found and had been using to move between the attic, kichen and basement.
About Muppet Babies

Title: Muppet Babies
First Air Date: 1984-11-05
Last Air Date: 1990-12-29
Status: Ended
Rating: 7.958/10 (from 119 votes)
Language: EN
Seasons: 7
Total Episodes: 107
Network: CBS
Genres: Animation, Kids
Production Companies: Jim Henson Productions, Marvel Productions
Synopsis
The Muppet Babies (Kermit, Piggy, Gonzo, Fozzy and company) live in a large nursery watched over by Nanny. The babies have active imaginations, and often embark upon adventures into imaginary worlds.
Cast

Frank Welker
Baby Kermit / Baby Skeeter / Baby Beaker (voice)

Laurie O'Brien
Baby Piggy (voice)

Russi Taylor
Baby Gonzo (voice)

Greg Berg
Baby Fozzie / Baby Scooter (voice)

Dave Coulier
Baby Animal / Baby Bunsen (voice)

Katie Leigh
Baby Rowlf (voice)

Barbara Billingsley
Nanny (voice)