iPhone Memory Full Message

My sister recently got a pop-up up her iPhone telling her that her phone’s memory was full when it clearly was not, so I googled this issue and one of the answers led me here. Unfortunately, I couldn’t reply to another thread on the same subject, so I’ll attempt to mention was I discovered here. The pop-up my not-terribly tech-savvy sister was seeing appeared when she was using a third part weather app. Whether it was from ad inside that app, I don’t know, but I thought I’d share that there is more than one “cause” of repeated iPhone memory-full messages.