Units should not automatically attack aliens if moved into an unexplored hex. I try my best not to antagonize the aliens in the beginning, but it is frustrating when I move a unit to a seemingly empty hex only to find out that an alien was there (usually for units 2 hexs away). I can understand you automatically attacking aliens, if you know there is something there, however, you should be given a warning if there isn't. Especially for units like explorers that aren't meant for offensive combat.