BoaT is not fully as CoO became later on. It was implied just to sense THINGS not living beings.
Laffitte has a cain, a hat, shoes, and has clothes which aren't supposed to be at there at the window. Surely that is suspicious enough.
So how does BoaT, which surely Mihawk had it, debunk Laffitte's infiltration skills?
If Mihawk sensed Laffite before he spoke, pretty sure Mihawk's panel will be shown first, noticing Laffitte before speaking.
That's not what happened.
Laffitte was already at the window, spoke, then Mihawk noticed.