Many Grateful Dead studio CDs have "hidden" tracks. These are included as separate tracks, not listed in the liner notes but playable if you step forward through the tracks. They have varying lengths of silent introductions. You'll want to let the CD run on from the previous track.
Thanks to whitegum.com for compiling this list. What new tracks can you discover from other box sets? Post anything new you find in the comments on our FB page here. https://www.facebook.com/mickeyhart