A magnet will not be attracted to or find copper. A metal detector, or a cable and pipe locator would find a copper pipe. Long shot - If its a plastic pipe and run some water through it, hot or cold, you might be able to spot the temperature change in the plasterboard, with an IR thermometer.
Using a small flat-bladed screwdriver wiggle it through the plasterboard to form a test hole at the position you want to place a fixing. Repeat for other fixing locations. If upon penetration of the plasterboard you cannot feel a pipe then using a drill CAREFULLY enlarge the hole to the required size. It's unlikely that the pipes will be tight against the back face of the plasterboard - but you never know.
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