without some more detailed description, we can only guess...
But my guess would also be, like raceprouk already partially said,
- In Messenger:
Tools > Options > Personal > My Status > Show me as "Away" when I'm inactive for x minutes"
Tools > Options > Personal > My Status > Show me as "Busy" and block my alerts when I'm running a full screen-application"
- In Plus:
Plus > Preferences > General Options > Miscellaneous > "Automatically change status after period of inactivity"