Indeed there is Footbowler, according to my old Grandad, and not many people know about it.
The only access point to the tunnel was had from the Mary Queen of Scots Room in Castlemilk House via the fireplace.
The secret passage could only be gained by removing the vase next to the clock on the mantelpiece, then the fireplace would spin round in a james bond type fashion to reveal the steps down to the secret tunnel itself.
Here is a picture of the interior of Castlemilk House featuring the very fireplace itself in the MQoS room.
This is a view of Castlemilk House from one of the air vents further down the hill.