Cold water leaking into the hot lines?
Do you have a single handle faucet? 

Most single handle faucets /showers will leak over from cold to hot or from hot to cold during use (if they aren't set at the absolute max/min setting).  To prevent this try turning off the local hot water valve at each single handle faucet/shower.

Do you have mixing valve somewhere, like for a bidet system? That would also likely cause a similar problem.

Otherwise it could just be the draining of the remaining water from the hot water pipes at a very slow rate.