I had one machine provisioned at the time of a Jenkins restart (upgrade from 2.7.1 to 2.7.2) and when I restarted Jenkins it reconnected to the machine.

After the retention time had elapsed, the machine stayed around. I checked on some connectivity times this morning and it has now been far longer than the retention time of 15 minutes.

import java.util.Date

println new Date(Jenkins.instance.getComputer("Azure0808081943").getConnectTime())
println new Date()

prints:

Mon Aug 08 21:43:28 UTC 2016
Tue Aug 09 17:38:10 UTC 2016

So this machine has effectively been online for almost 24 hours with nothing to do!

uptime from the machine also corroborates this story: {{ 17:43:25 up 21:22, 0 users, load average: 0.33, 0.11, 0.07}}