On of our AP MR56 is showing usage on the 2.4 Ghz as 80% usage but no device connected
Any way to work out why such high utilisation
Channel utilization is an indicator or interference and is unrelated to the number of associated users.
In your scenario, your AP is broadcasting on channel 6 and is indicating that there are other devices within range broadcasting at that same frequency, thereby causing interference.
The AP is showing how 'airtime' is being consumed locally, within channel 6 and the immediately adjacent channels. Bear in mind the AP itself will be generating some of that traffic, even without associated clients - in regular beacon frames, advertising each of the configured SSIDs. But, unless you have many, many SSIDs, this will be other nearby devices (APs & clients) generating traffic in the same channel space. If you have a look under Wireless > RF Spectrum, select the AP in question and click Interfering APs, it will show you nearby APs with which your AP is competing for available time. Bear in mind too that not all consumption is wifi based - as the oldest, lowest frequency unlicensed chunk of spectrum, it's used by plenty of other technologies, such as Bluetooth. Some of the energy can be non-transmission based too (famously; microwave ovens).
You could look to move to an alternative channel (1 or 11) but AutoRF does this for you, as best it can within the small amount of availble spectrum in 2.4 GHz. You may well find you're better off with clients using 5 GHz as much as possible, by using Band Steering. https://documentation.meraki.com/MR/Wireless_Troubleshooting/Wireless_Issue_Resolution_Guide#Poor_Th...
Also: this is one of the major reasons why regulatory bodies, across the globe, have released more spectrum in 6 GHz. The industry, including Meraki, has responded with new AP models, with an extra radio to also handle those new channels - WiFi 6E being the buzzword to look for. You will need new clients to take advantage of it, but past history tells us that new models of laptops, smart phones, tablets etc. will adopt it quickly. Make sure whoever procures these things in your Organization look to get 6E on their desirable (essential?) feature lists - particularly if key locations see channel over-utilization such as that you show here. In the meantime, for Meraki 6E APs, at least, you can use the 6 GHz radio as a second 5 GHz one. Select the WiFi 6E filter on this page, to see the options: https://meraki.cisco.com/products/wi-fi/models/
If there are interference sources on channel 6 or just a whole bunch of neighboring access points on channel 6 or even worse overlapping channel 6 then you can only avoid it by not using the channel. If possible and all your devices support 5 GHz or 5/6 try to avoid 2.4 if possible. Perhaps restrict certain SSID's from transmitting on 2.4 GHz band.
What do we mean by Airtime Utilization? Free airtime describes how much time is available on a channel for data transmissions. Any time a device communicates with another device or an access point (AP) it uses airtime. Airtime Utilization is a per-channel statistic that defines what percentage of the channel is currently being used, and what percentage is therefore free. Airtime usage can come from: