As you approach the equipment, you will of course hear the sound increase in volume and pitch. If there aren't any targets, it will be smooth. However, if there's a target, the sound in Auto-tune will be different, depending on how the target lies, whether it's tilted or not, etc. Usually, the sound jumps up faster than normal, levels off for an instant, then continues swelling in volume and pitch as you get closer to the equipment.
This takes getting used to, but it does work. I pulled a small 10K gold ring 4" from a support leg of a piece of playground equipment once, as well as coins.
If you still can't hear a target, and you're in a woodchip or sandpit playground, rake out the suspect material, scan it with your detector, then replace it after recovering any targets.
I've found the most bleachers are just to junky and/or the supports are to close together to effectively detect. Build a sifter and sift the dirt. You'd be surprised what you'll find.
HH from Allen in MI