Just because you cannot hear it does not mean it is not there. This tool scans the ultrasonic band - 15 kHz up to the Nyquist limit of your file - for narrowband high-frequency content that survives inside ordinary recordings, using a short-time Fourier sweep with per-bin prominence detection (a tone counts only when it stands clearly above its local spectral neighborhood, so wideband hiss does not false-positive). What it looks for, with the frequencies the literature actually names: ultrasonic TRACKING BEACONS in the 18-20 kHz band - the cross-device tracking channel documented by Arp et al. (EuroS&PW 2017), where TV commercials and in-store audio embedded inaudible identifiers picked up by phone apps carrying the SilverPush, Lisnr and Shopkick SDKs (Google Play removed such apps in 2017) - a steady tone here in a broadcast capture is worth knowing about; the classic 15.734 kHz NTSC flyback-line whine (and the 15.625 kHz PAL sibling) that CRT-era and badly-shielded gear leaves on recordings; ALIASING IMAGES hugging the Nyquist limit, the fingerprint of a careless sample-rate conversion; and short transient HF pings. Each detected tone is reported with its frequency (10 Hz resolution), start and end time, prominence in dB, duty share and a classification. The privacy angle is real: if your upload contains 18-20 kHz identifiers, that recording can link devices and locations across time. The tool needs a source sampled at 32 kHz or higher to have an ultrasonic band at all; 48 kHz sources cover the full beacon range, and heavily lossy encodes (which low-pass around 16-20 kHz) may have already erased the evidence - a negative result on an MP3 is not proof of a clean original.