android - "Exported receiver does not require permission" on receivers meant to receive from system services

I have some receivers declared in my AndroidManifest :

<!-- no warning -->
<receiver
    android:name=".receivers.TriggerMonitoringBootReceiver"
    android:enabled="false">
    <intent-filter>
        <action android:name="android.intent.action.BOOT_COMPLETED" />
    </intent-filter>
</receiver>

<!-- no warning -->
<receiver
    android:name=".receivers.ScanResultsReceiver"
    android:enabled="false">
    <intent-filter>
        <action android:name="android.net.wifi.SCAN_RESULTS" />
    </intent-filter>
</receiver>

<!-- warning : Exported receiver does not require permission-->
<receiver
    android:name=".receivers.BatteryMonitoringReceiver"
    android:enabled="false">
    <intent-filter>
        <action android:name="@string/intent_action_setup_alarm" />
        <action android:name="@string/intent_action_cancel_alarm" />
        <action android:name="@string/intent_action_monitor" />
    </intent-filter>
</receiver>

The first one is meant to receive a BOOT_COMPLETED action. The second is meant to receive android.net.wifi.SCAN_RESULTS. The third one is meant to receive some actions I broadcast (intent_action_monitor) and some actions broadcasted by the AlarmManager (intent_action_setup_alarm etc).

Two questions:

• Why don't I get the warning on all receivers?
• What permissions do I need to set for receivers meant to receive from system services to correct the warning (I understand what it is about and I don't want anyone to use my receivers anyway) ? Will exported="false" do for boot receivers, wifi receivers, alarm receivers etc?
  I thought of using a custom permission with android:protectionLevel="signatureOrSystem" but the docs advise against bot