I ran the plugin update for the SimpleSearch plugin, and it broke the page, and seemed to disappear from the plugin directory. A poke around the files showed that the simplesearch directory contained the files for the Github plugin (and the github plugin showed twice in the directory).
Running bin/gpm install simplesearch pulled down the same files. I resolved the issue by downloading from Github directly and copying the files across. The admin is still looking to upgrade simplesearch to "2.0.0" although latest dev version is 1.14.2....
The actual code (for both plugins) seems to be OK, although they were both updated by the same user at roughly the same time? Wondering if this is something more to do with the directory than the plugins?