I was looking at that "Sonic Grenade" project on the internet and I tried it once. The 110db keychain alarm sold at the dollar store is perfect for tripwire. It has a pin-based switch so the alarm rings until you put the pin back in the hole. It varies in shape (mostly oval or square) and color (translucid blue or red) but they are all the same and are clearly tagged "110db personal alarm".
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To make one, simply buy an alarm and attach a strong but thin wire to it. Place it where your ennemy should come and tell your teammates. When the alarm sounds, the ennemy's stealth is lost and you all know his position.
I'm about to buy one today and try it. We play mostly after 6pm so the wire will be hard to see.
*evil laughter*