If you’ve been on a conference call where the mic was in the middle of a long table, you were probably hearing a boundary microphone. They’re flat, they hide on surfaces, and they’re purpose-built for groups and rooms instead of individuals.
A boundary microphone (sometimes called a PZM, for Pressure Zone Microphone) is a small flat condenser capsule mounted facing a flat surface — a table, a stage floor, a wall. The surface itself becomes part of the pickup, doubling the effective surface area and giving a half-spherical pickup pattern instead of the usual cardioid or omni.
If you’ve ever been on a conference call where the mic was in the middle of a long table picking up everyone, that was almost certainly a boundary mic. Theater productions use them along the front of the stage to pick up actor footsteps and dialogue. Sports broadcasts use them on hockey boards and basketball backboards.