Because your log statement is all kinds of wrong. What are you trying to log out with gAllAlarmSensors.lastUpdate("PIRDetectorSalon")? It is pretty close to nonsensical.
Your next line is the correct way to work with lastUpdate.
If you ever want to pass a String to lastUpdate, it expects the name of the persistence engine to use. This lets you query for data in your non-default persistence. That is what the error means. You have no persistence engine named “PIRDetectorSalon”.