I've setup the cloud to have an instance cap of 20 and 10 minutes retain time.
Made sure that the list of nodes had only master and that in Google Cloud there were no VMs.
I've then launched a job that fans out in lots of different jobs which forced the plugin to provision VM instances.
The plugin ended up creating 20 instances in Google Cloud but only 9 were listed in the Jenkins nodes page.
I then waited for 10 minutes to check whether the plugin would have kept track of those VMs. Unsurprisingly 11 VMs in Google Cloud were not terminated.
This seems quite a nasty bug cause it would lead to unnecessary costs if those orphaned VMs are not spotted and terminated manually.