Is com.apple.madrid.plist a suspicious plist?
"Madrid" was the internal codename for iMessage before the iMessage product name was used.


The above is from @pytey on Twitter. You can find many references to Madrid within iMessage, many of which are detailed in Messages on theiphonewiki.com.

Even if you don't use iMessage, the plist still exists as part of iCloud communication. It stores simple metadata, mostly around syncing and CloudKit per the keys in the file.