If police are advised that a burglar alarm has been "set off," does that in itself mean police can intrude into the home to determine the cause of the alarm, absent evidence to the contrary?
If a police person has probable cause to believe a crime is in the process of happening then he or she can enter premises without a warrant.
Under the exigent circumstances doctrine some people must be under danger, evidence must be in danger of being destroyed or a suspect must be fleeing.
If these circumstances have been proven then plain view doctrine applies to any evidence or contraband seen in the process on which these circumstances hinged.
It is probably safe to assume that a police officer responding to an alarm is justified in believing some people may be at risk of harm.
Probable cause is considered at the time of entry. There are a myriad of things that can lead to a false alarm but the police person cannot know this unless he or she investigates.