Virtual machine keeps starting on Teamcity request

We have had 3 VMs running without problem for a long time, In the last few weeks however, one of our VMs continually starts and when is manually shut down will start up again immediately. If I disable the build agent running on the physical machine that this VM is running on then the VM does not start so it's definitely a request from TeamCity that is starting the VM, however no builds start on this agent (once it's started) and the history of that agent show no builds.

The VM agent can succesfully run builds when they are requested. It is like the VM permanently has a request from TC to start up!

When I look at the teamcity-clouds.log the agent that keeps trying to start shows

[2018-01-30 13:48:12,062]   INFO [2 Flush Queue 1] - .server.impl.CloudEventsLogger - Cloud instance entered 'scheduled to start' state, XXXXX

What is causing TC to "Schedule this agent to start"? 

Any suggestions much appreciated on how to debug this?