Certainly @BlakeRichardson, if needed. First, @Dark_Knight did you call into Meraki Support and tell them you want to have this feature turned on, if you do not already have it? It would be under Systems Manager > Monitor > Overview if you already have access to it. If not, the first itme under the Monitor column would be "Devices" instead of "Overview". If you don't have "Overview", give Support a ring to request it. If that doesn't work, drop me a PM or get with your local Meraki reps to investigate.
Once it's there, as @PhilipDAth mentioned, you will have a graph that shows which devices have been offline (not checked in) in more than 1 day, 3 days, 7 days, 30 or 90 days and these are clickable links, for convenience, that then use the connected_at filter on the Devices page. For example connected_at:3-7 to list all devices that have been offline between 3 to 7 days. I would also make a wish to make that Overview page exportable, and schedule-able so if you wanted you could get daily or weekly emails.
You could also investigate leveraging the SM API and write a script to pull this info using the GET command /networks/[network_id]/sm/devices in the SM calls, and one of the additional fields is lastConnected, but I haven't tried messing with this myself yet, maybe you'll be able to leverage that to automate something.
