Covered vent under a floating wood floor
Couple ideas come to mind:


Focus near windows, this is the typical location of an HVAC vent. Compare the room with others to find common locations, spacing from the wall, and typical number of vents in the room.
If you believe you have located it, a deep sensing stud finder may identify edges of the vent if you start in the middle.
Try loud sound/music directed into another vent.
Look from below the floor, making a small access hole in the ceiling below where you suspect the vent may be located.
Since the floor is floating, pull the trim and lift up the last few rows. The floor can be put back without any impact, and the trim will just need some caulk, nails, and touch up paint.