Hi rebow

Here is one of many possibilities. Since you have turned off both of your lines and an area around the sprinkler remains wet (do you actually see it leaking from the sprinkler head), you might have artesian flow coming from the ground. Take the sprinkler head off. If the lines are blown, there should be no water in the sprinkler head body or it may go into it from the surrounding ground. If there is water in the line still, it may be from a cracked lateral line that has water going back into it from the outside soil or your system has a main valve leak. It sounds like this head is in a low area that collects seepage from the entire yard. With what you described, it sound like your lines are not connected to your neighbors. I am making a lot of assumptions. One test, if the sprinkler head has water in it, is to sponge it out and watch where the water comes from. Check it out. GL Aloha.

Is the head on a slope? The lowest head on a zone will weap for hours after the system is shut off as the lateral lines empty in it.
Has he replaced the diaphram in the valve? Sometimes a pin hole is enough to let water slowly by but not open the valve all the way. The lowest head would leak in this case.
There has to be a reason. There is no such thing as spinkler magic.

I believe lehua13 is steering you in the right direction.

To really determine if there is a leak from the head and not something else (like ground water drainage), dig down as much as needed and remove the sprinkler head (this shouldn't require much digging as the TOP should most likely be able to unscrew allowing the "guts" of the head to be serviced without digging the whole head up out of the ground and disconnecting it from the pipes).

If there IS water comming up into the sprinkler head with both of ya'll systems shut down and no water meaters turning, then about the only thing that makes any sense to me is that he has a broken pipe that is somehow allowing water back-into the system after a blowout. After all, it is possible that a broken line has created a little cave underground that fills with water when he runs the irrigation system. When he then blows out the system, that little cave can still be filled with water. Then, with the system shut down, water from the little storage cave could be flowing back into the broken pipe and out the leaking head.

Thanks for all your replies, once the ground unfreezes here in 6b we'll look into this further. Any new idea welcome.

Sounds like the valve is leaking from the diaphragm. They can change out the diaphragm or replace the entire valve.

Here is a link that might be useful: leaky valve