A great aha for me from GTD is the "What is it?" step of processing. It may not be as obvious as it seems. It sounds like the potty alarm is also a subtle reminder of an upcoming project: Potty Training Younger Child. Is there a reminder somewhere that this project is coming? It could be in someday/maybe or on your calendar with a note about where the potty alarm is stored. That may be why it hasn't been put away before now. Some part of your brain is using the potty alarm as a reminder.

Also, I think about where I want to find the item later so I would put it at the bottom of the diaper storage or with the potty seat that is probably stored somewhere.