My domain is directed only to one version (non-www), we do not cache sessions. We make use of memcache and everything has been set up by @MattW
Memcache stats show lots of free space in cache, so cache isn't full or causing any issues (even though we don't cache sessions just clearing this out), and since only one domain is being used there shouldn't be cookie loss due to domain. Could this be any other issue that needs to be looked into server side? The log outs happen randomly, and until now it has only been reported via mobile devices.