The magnet is generally located on the upper edge of the display bezel. If you take a decent magnet and run it around the perimeter of the palmrest, you should be able to pinpoint the location of the switch itself. Also make sure you do not have any magnetic objects near the palmrest. I remember a post where the user's system kept going into sleep and it turned out that she had on a bracelet with a magnetic clasp.

One or another of those magnets does double-duty.  The switch itself won't be a discrete component -- it'll either be part of the mainboard or another board such as the power button board.

I've taken the laptop down to the case when I replaced the power button board and the only magnets I saw were 4 along the top of the display and 4 along the bottom below the palm rest. Also the only on the display have the north pole upward facing the the ones below the palm rest had the south pole facing up so I think they're to help keep the lid closed. On typical hall sensor setups the magnet has to be very close to the hall sensor to sleep the laptop but mine will do it with the lid at 45°. 

It could be double duty. I called Dell and they said they don't sell the part so I don't think it's a discrete board since the m18s. It could definitely be on the motherboard though. If that's the case I'd have to replace the entire motherboard or send it somewhere to solder on a new hall sensor.

I finally found it. It's between the power button and left hing at the top. There's a small magnetic in the lcd bezel. The sensor is actually on the board.