My guess to as what you are experiencing is that you have a larger then coin sized piece of foil. All metal detectors are designed to get the best accuracy for defining targets and their depth using coin sized targets sitting parallel to the coil. Any deviation to this results in often false readings. This is typical for all metal detectors. To help validate what the target might be, keep swinging the coil over the target, slowly raising it at the same time. If say at about 12 inches you still have a strong signal, it is a larger then coin size target. Your choice to dig or not.
This is good information to know about your detector, and there are many more tricks you will eventually learn as you get more hours. Below is another trick you can use to help with old bottle caps.
"I accidentally discovered a way to distinguish old rusty bottle caps from coins also reading as bottle caps. I was hunting some real old parks, just loaded with old rusty bottle caps several years ago, and was ready to give up as many others had. Something was strange about those bottle caps? Once they were out of the ground, most would remained sounding like bottle caps. But, the odd one, after missing it the first time with my digger trying to retrieve it, changed from reading as a bottle cap to reading as a coin, either copper or silver. Why? My guess is that the copper or silver coins over time developed a layer of oxides around itself in the soil matrix. Some people have called it the "HALO EFFECT". And to this I developed the "STOMP'N METHOD TO ID RUSTED BOTTLE CAPS"
If you are hunting in an infested area with old rustly bottle caps, stomp your foot above the target "HARD!" If it's a bottle cap, it will continue to sound like a bottle cap. If it's a coin however, it will now read as a coin, so dig it.
