Purchased songs disappear from library

Playing random on iPhone, I noticed that I hadn't heard a few songs in a long time. Search library on the phone, not found. Search the library on iTunes, not found. Search the location of downloaded files, not found. Search in iTunes store, it says 'Purchased' (see picture below), but it just says 'Purchased', no option for 'Download' (the button is gray, cannot press it). Account (menu), 'Check for available downloads...', says 'There are no available for (my account name)'. This happened once before several months ago, with a single song. I found the song again in iTunes Store. The button for the song was active, with price (not 'Purchased'). I clicked it, then it said 'You already purchased this song, would you like to download it?'. So I was able to recover that one, but not the recent ones in a similar fashion. Questions: * Any reliable way I can find if this has happened to other songs? (My library is quite huge, I don't have a solid record of what I think it should contain) * How can I recover any songs this has happened to? (I prefer to download the files to the iTunes computer)