I'm usually using Chrome on an iphone and Chrome on a Windows 7 PC, but it happens with Safari on the iphone and Firefox (latest version) on PC as well.
If I come out of Chrome/Safari to do something else on my mobile, or close the browser on my PC accidentally and not to hide MNet coz I should be working it always logs me out. I've checked that I've got "Remember Me" ticked and I'm not set to delete cookies/history/cache.
I could understand if it logged me out on my phone when I use my PC and vice versa, but this doesn't seem to be the case - I can be logged in on both, but close Chrome and "bye bye MNet"!