It doesn't weigh anything.
It's an "invisible, mindless, shapeless force", not a corporeal being. It interacts with its surroundings by grabbing or pushing on objects, which it does with a Strength of 2. This gives us some idea what it can do: assuming it works like a Medium creature*, it can carry 30 pounds and push or drag 60 pounds.
Since it's insubstantial, it couldn't "stomp" on a pressure plate to set it off, but it could push against the floor with 60 pounds of force. This should be enough to trigger any trap that would catch, say, a gnome.
A more efficient use of its strength would be to push a handcart full of rocks. The weight limit for a wheeled vehicle is five times your carrying capacity, so it can push 150 pounds, which should trigger any pressure trap that a single humanoid could set off. This will also detect tripwires, since the cart's wheels are touching the ground. Of course this requires you to drag a cart around, but if it survives its use as a trap detector, you can also use it to haul treasure home.
*It's not a creature at all, of course, though the spell does compare its range of possible tasks to a "human servant". Absent any other indication, I'd rule that it uses the same size class as its caster, so that it's useful in the same way whether it's a pixie or a giant casting it.