Is there an internal log that can be retrieved or is there a way to inject a webhook that fires (updating a log engine) each time the channel URL receives an HTTP request? The reson should be easy to figure out - analytcs, I want to see where the requests are coming from, headers, volume and when (date/time).
ThingSpeak does not currently provide this information. If you are concerned about the access to the channel, make it private. This way the only people accessing the channel field is those who you have shared the read API key with.