I'd like to close the tube that from my garden go to the garage box. Is there any cheap way to figure out where the tubes are without dig the ground/walls?
Here is what I would do to locate that plastic pipe under ground - I would disconnect it at your garage box, I would then insert a copper, aluminum wire of sufficient gauge to be able to feed into the pipe with out it curling up on you or you could run Electricians steel 'Fish Tape' .
At this point use a metal detector - you can buy them for about $100 - $200 U.S.
Now using the metal detector you can plot where that Plastic Pipe is located ..Very easily..
can't you call your utility company to mark where things are underground?