Mayday, mayday!
This is a universal distress signal. It's usually radio.
"Pan-Pan" is used when a problem arises but no one is in danger and calling "Mayday" would be overkill.
Your pilot should never mention this number. The code 7500 indicates a hijacking.
Code Adam
Malls, airports, museums, and hospitals use this one. It means a child is missing.
Time Check
If you hear this in a store, leave immediately. A bomb threat.