I’m not able to recommend any third party software to monitor a iPhone, or scanning software to detect if there was remote access to your phone, sorry. Perhaps another member can direct you with those requests. My privacy approach is to stay on top of all available privacy settings the iPhone iOS provides, and I keep the phone security updates current. Like you, I don't want to feel vulnerable that my phone information is being accessed.
Facebook and your microphone button “on”…I tend to believe that the microphone setting was already on. Perhaps not comforting at this present time. I had no clue what settings each app controlled until I spent time in privacy settings. Some apps used my microphone and camera, others used my contacts and calendar, and so on. I was shocked to see how many buttons were ON!
From my experience owning an iPhone, a huge majority of settings are turned “on” by default. Unless you methodically take the time to look at every nook and cranny within the phone settings, you don’t know what settings are on or off. These default settings can change with every update <as with the current iOS 11.0.3> or with every new app downloaded and installed, so it takes constant checking (IMO)
Steps to take to review privacy settings <Some or all of these settings you might already do>
Click on -> phone Settings button <no particular order below>
-> Methodically go through each item/app listed to make any setting changes <many settings you might not know existed> <Like what you found with Facebook>
Click on -> Privacy
-> Click on each item in the app list to verify what setting(s) you want turned on or off. <Yes, it can take some work (with 45 apps!) but I find it’s worth it.>
-> -> with each app you will see a note in grey: Applications that have requested access to your <app> will appear here. Obviously if you see something showing of concern, then you take action.
-> -> below the last app on the list, you’ll also see in grey: As applications request access to your social accounts data, they will be added in the categories above. <Keep an eye on this app list to stay up to date with any new settings>
-> These two access notifications will hopefully allow you to feel comfortable that you're not being peeped on by an app.
-> Scroll down to the bottom of privacy and you’ll find: Advertising -> Limit Ad Tracking: ON -> Opt out of receiving ads. -> Click on View Ad Information, read the information -> click About Advertising which will redirect you to Apple where you can read more information about advertising -> Scroll down to the bottom of this discussion where you will find directives as to how to Choose Your Advertising Preferences (opting out of ads for example)
Click on -> Personal Hotspot
I keep this setting off. If you have it on, your iPhone Internet connection is shared, which I feel leaves the phone vulnerable.
Click on -> iTunes & App Stores
-> In-App Ratings & Reviews: I keep this off. For privacy purposes, I have no need to share my opinions with developers or other users.
Click on -> Siri & Search
-> My Information, if you don’t want Siri to access your contacts, change to none
-> Siri Suggestions, read the About Siri Suggestions & Privacy**great information -> <Personally I have Suggestions in Search and Look Up: Off.>
-> **Look at every app in the list to determine what you want Siri to learn about you.**
Click on -> Safari
Check privacy settings. (AutoFill, Block Pop-Ups, Ask Websites Not to Track, Fraudulent Website Warning, Check for Apple Pay: Off) Note: Camera & Microphone Access is found in Safari too. <I keep mine off>
Good luck mastering your sense of privacy with your iPhone.