There are few services like the one you mention that are popping up which would certainly be an extra way of monitoring.  

Because you get automatic alerts to your phone if it gets too loud (and you can set hours & noise levels), it's a lot easier to monitor than a camera system. 

Do you charge for monitoring and maintaining vacant units?

Bigger Pockets has some great state forums and usually there are good locals that monitor those.

I'm wondering what system you'd  recommend for remote monitoring and security for the units?

Best thing to do is to monitor it.

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@Devan Mcclish When you sub-meter you need to figure out who will actually be monitoring the meters.

@Brady Jones We signed an agreement with the company that puts in the equipment and does all of the monitoring/ repair of their equipment so they have the password/user agreement with the residents.

Use a map whenever possible instead of a GPS so you can remember the area#2.)