Improve Provided Code

Simplisafe's alarm service evaluates a batch of sensor events (door/window sensors, motion sensors) to decide whether to trigger an alert, then formats a message naming which sensors fired. A teammate wrote the two functions below and asked for a second pair of eyes before it goes into the alarm pipeline.

def evaluate_alarm(events, triggered_sensors=[]):
    for event in events:
        try:
            sensor_id = event["sensor_id"]
            status = event["status"]
            if status == "open" or status == "motion":
                triggered_sensors.append(sensor_id)
            if len(triggered_sensors) > 3:
                return True
        except:
            print("bad event")
    if len(triggered_sensors) >= 3:
        return True
    return False


def format_alert(sensor_ids):
    msg = "Alert triggered by sensors: "
    for i in range(len(sensor_ids)):
        msg = msg + str(sensor_ids[i])
        if i != len(sensor_ids) - 1:
            msg = msg + ", "
    return msg

Given this code, how would you improve it? Point out anything you think is actually wrong, not just stylistic, and explain the impact each issue could have on a real home security system.