May I point out that if the humans anticipated the attack at all - ie if they saw the ship coming towards the base... then the base would be in lockdown mode - and I wouldn't expect all the entry points to be open? I'd expect a congregation of aliens outside the entry portals trying to cut or blow their way in.
I'd also expect some kind of base security in place for precisely this reason... in 2084 perhaps automated defense systems might be plausible.
Of course if they somehow sneaked in under the radar, then yes, I'd expect the portals to be open, and all my soldiers to be out of uniform on the sun deck by the pool / in the bar / mess / rec room and only two or three sentrys on guard.