(something) is gathering dust

When you don't use or move something for a long time, it starts to get a lot of dust on it. So things that don't get used are said to be "gathering dust". Examples of things that might "gather dust" in your home are:

• unread books
• strange kitchen appliances
• rackets, weight sets, and other sporting equipment

There's also a similar phrase, "collecting dust":

It's just collecting dust in a closet somewhere.

a (something)

Notice how the speaker says "a closet". That means that she has more than one closet, and she's not really sure which closet the waffle iron is in.

(somewhere) somewhere

When you're not sure about where something is, you can say that it's "___ somewhere". For example:

It's over there somewhere.

I think she lives in the U.S. somewhere.

You can also say this in the order "somewhere ___":

It's somewhere over there.

But "___ somewhere" includes a small sense of not caring where the thing is. In other words, you're saying "It's near ___ but I don't really care exactly where it is." "Somewhere ___" doesn't have the same nuance.