Found that last alien!

[CampDragon]

 Terror mission.
 Turn 118
 no losses on my side
 however was down to that last pesky alien from Turn 22 onward.
 Finally found him behind a stairwell, in an area that is not accessible to walk to or shoot at. Grenade got him. 
Got to the point i was NOT going to dev mode or cheat it. Was chuckling to myself from turn 70. Before that i was mad. When i got him i was just relieved I found him from just random grenade throws while seeing if i could just bring all the buildings down.
So anyways we have all been there, but can any of you remember the longest you have spent looking for that last one?

[SIMON BAILIE]

I think if you play openxcom extended+ and reach at least turn 20, the game enters bug hunt mode which means all remaining aliens appear on the minimap.

[CampDragon]

 That is a nice feature. Thank you for the heads up. But I was not playing that version. Sad face.

[KZad Bhat]

It's starting turn 16, maybe 15, if there are no more than two baddies. It's also alterable, but I can't recall where I found it.

[Meridian]

Details here: https://openxcom.org/forum/index.php/topic,4997.msg73330.html#msg73330

[The Reaver of Darkness]

Longest I went was about 30-40 turns in a TFTD ship attack top section, on the cruise ship. Turns out I had stared directly at the alien twice and not seen it. It was a bio-drone hiding (or stuck?) in a corner in the crew cabin, in which an air block was occluding it, making it invisible where it should have been visible.

[Yataka Shimaoka]

Highest I have is 228 in tftd vanilla cargo ship part 2

[R1dO]

228  .. really??

There exist people that manage to finish the game in that many turns (not me though).

[Meridian]

[The Reaver of Darkness]

There have been a few occasions in which I was hunting for that last alien and waited until turn 20 so they would come to me, but they never do. I think the Leeroy flag is being subverted by the node prisons. In one particular case, I discovered a sectoid hiding upstairs in one of the base facilities that isn't supposed to spawn aliens.

[Meridian]

Turn 20 doesn't mean they have to come to you... it just means they scout "more actively".

Same for vanilla AI and for Leeroy AI.

Btw. Leeroy AI is very simple... if you have a melee weapon and you can see an enemy... attack (even if you can't attack, i.e. it will take you more than 1 turn to attack)... otherwise patrol. No ambush and no escape.