On top of the numerous calls and timed missions, several secret missions can appear then run out without you even knowing. Here is a list of the secret missions so you don't miss out:

1 // Inside the Yucatan, before you meet Rebecca on Day 1. An optional boss fight is here, along with a survivor hiding in the offices. Head on over to the Food Court and raid it for Steak, and ONLY Steak. Upon entering the Yucatan you will be greeted with a cutscene. Afterwards, beat down Ted, watch him die, then hop up on top of Snowflake's house. You should find 3 steaks total around her enclosure. Toss them way out there and wait. You'll know when she's eaten some because her health bar will have filled a bit. Just remember to stay out of reach, otherwise she'll use YOU for lunch. After about 6 Steaks you'll be greeted with another scene. Finally, save the poor guy trapped inside the Yucatan's office.

2 // Royal Flush Plaza, Casual Gals store, Day 1. After rescuing his wife located directly in front of the Safe Room hallway, Gordon will appear here and need your help getting back.

3 // Royal Flush Plaza, upstairs, during the Rock Heroes mission. While escorting the band, swing by up here to see 3 girls doing their shopping. To get them to follow you have to carry their delicate shopping boxes back to the Safe Room.

4 // Americana Casino, VIP Poker Room. Starting midway through Day 2, some high-rolling poker players will set up a game here. To save them, you must pay the $100,000 buy-in then win. If you lose, you have to pay $100,000 AGAIN and keep trying.

5 // Food Court, after the Tastes Like Chicken boss fight. So, you've killed the psycho and untied the damsel, but she thinks you're a terrorist and won't come with you unless Jasper shows up. He happens to be in the Food Court too, but in an unlikely place. Find the nearest vending machines and get to climbing. You'll find him on one of the many restuarant awnings. Re-unite the pair and hoof it home.