Tanya,
I run
www.LostandFoundRing.com ... and have experience with this. Depending on the type of hearing aid, these can be some of the hardest items to retrieve - because the only real metal in most modern hearing aids is the battery. A traditional metal detector likely won't even pick it up. I have had success using a small "beer can coil" attachment, and even then it can be very hard if there is any other metal debris or interference in the area.
Feel free to reach out directly to me if you'd like some help or guidance. I'd waive any search fees for friends of Sector67. Depending on what side of town you are on, might determine how quickly I could come out and look myself. If you have the other one, we could see if a small detector, called a pinpointer might register - if the search area is small. I'd loan this out to you for free. Sometime the best tool is a large magnet, like a contractor magnet on wheels or a "magnet fishing" magnet. I have the later and could borrow this out as well. All depends on the search area, type of hearing aid and battery.
Happy to help how I can,
Dan Roekle