Do personal locator devices actually work?

I’m not sure if this belongs here or in GQ, but since what I’m looking for is more in the lines of personal experiences, IMHO it is.

The backstory (what there is of it) is this: My father was recently diagnosed with Alzheimer’s. He and my mother are still active and generally able, but she’s started “loosing” him. He’ll wander off in a public place (which he’s always done, actually) and then forget where he last saw her. Then he’ll just hang out wherever he happens to be until she finds him again.

I did some cursory searching to see if there was any technology available that might help her find him again (particularly in higher traffic locations such as airports), or, failing that, give her some peace of mind if he wanders off in a more dramatic fashion than getting separated from her at the grocery store. I was looking at the kiddie locator things, but I’m not sure if those would be the correct choice or not. What I’m specifically looking for is some form of location device that’s (ideally) small enough to be carried or worn at all times. Other than a cell phone. My dad refuses to carry a cell phone. Maybe a keychain deal, or a credit card size that could go in the wallet.

Thanks in advance.