"This is the one time we had to cheat.
Throughout most of development, NPCs were able to see Ellie. So, if she ran out of cover stupidly, she'd be seen, she'd break stealth, a search would start, gunplay would start. This was great, it pushed us to polish the covering and following systems as much as possible. But ultimately, a few weeks before shipping, it came down to a decision: either we were going to be true to our "no cheating" rule and Ellie was gonna give you away, or they were not gonna be able to see her out of combat. Ultimately, we favored the player, as we didn't want them to hate Ellie.
There was further work we could've done, but we only had 5 months to program her AI. We had a couple of potential solutions in the works: we tried having "exposure maps", which basically are what the NPCs can see at any time, and we'd have Ellie never run across an exposed area, but it didn't fix much; we toyed with very soft teleporting in very specific situations when the player couldn't see it and she was really far away, but we moved away from it; we also tried a system where enemies would look away from Ellie if they were approaching her, but this had the side effect of changing how patrols worked and created more issues with the enemy AI. Ultimately we decided to polish the whole experience instead of reworking systems so close to release."