Someone was quicker than you!By this time, I've noted where all the outside guards are. I get to a terminal inside the building (IND) and locate the locker. I creep round to it, open it with the relevant code and... there's no chemical sample. I just grab everything anyway. Nothing.

It's annoying when the mission item is not there. In my experience, when that happens, even the mission pop-up message (top right) that points me towards the location is bugged, however.
Many times I've had a mission NPC missing from the settlement, however.
You can try relogging, though: while it's true that all the other lootables will disappear, the mission item should still spawn (sometimes at a different location).