It’s your job to assess the situation using your available resources. A beeping noise is not ‘a possible bomb on board’. It’s a beeping noise. Guess what beeps on board an aircraft ? Air Tags. They beep in lost mode, they beep when your apple device thinks you’re being tracked by one, when they’ve lost a connection and when the battery is going to fail. I’ve had my phone tell me I’m being followed by an AirTag - it was in the baggage compartment and I was in the cockpit, at FL410. Numerous other devices beep.

I’m not questioning the decision making of the crew. Maybe they fully assessed the situation; searched the aircraft and contacted operations, who then spoke to corporate security, who then advised them to evacuate if no other method for deplaning was immediately available.