Back then? Well... If it's a short episode - it happened more often; people fell in love and forgot the time, alcohol was also reason enough to forget the service... - curfew, possibly arrest (if it happened repeatedly), exclusion from a promotion round, possibly demotion. However, if it was a matter of desertion, i.e. wilfully leaving the unit and tacitly abandoning the service: violation of the oath of allegiance led to immediate dishonourable discharge. In the event that it happened during a mission, the death penalty still applied for a while under military law. However, this was no longer enforced; moreover, there were no missions for our army. So in fact nothing could actually happen ;-)) Today... nothing happens.