I've tried searching AO3 and googling, and haven't had any luck, so I guess either I'm failing to hit the right keywords, or I'm way off base and I'm actually thinking of, like, a Sentinel fic or something (which would be embarrassing, so fingers crossed).
Any help is greatly appreciated, even just reassurance that I haven't dreamt the whole thing. Thanks!
ETA: Found, thanks to michelel72! It's Phil Coulson Plus One.