I've seen a spike in my channel or connection usage that I wasn't expecting

Make use of the stats api (https://www.ably.io/documentation/realtime/statistics) to see when there's bursts of activity which should allow you to start collecting some information about what might be causing the spikes.

You can make use of channel enumeration to see which channels are currently active, and thus which channels are potentially responsible for a spike (https://www.ably.io/documentation/rest/channel-status#enumeration-rest).

It's a good idea to have a catch on devices using Ably, where they detect themselves if they're publishing over a certain threshold, and send a message with details to one of your servers if so.