[SleazeyWombat]

Nope, it really was the receivers.

The old tube receivers, and the tuning circuits they used (super-heterodyne) actually had a radio frequency oscillator as part of the circuitry. It helped the sensitivity and selectivity of the receiver.

However, a sufficiently sensitive U-boat radio receiver, tuned to the target ship's superhet intermediate frequency (IF), could pick up the superhet's inadvertent transmissions. A skilled U-boat radio operator could then home in on the merchant ship.

The IF frequency was constant; it didn't depend on what station the sailors were currently listening to. I do not know if all brands and models of superhet receivers used the same IF. The IF used was an engineering decision, not an FCC standard, so it probably differed from brand to brand, or model to model. Obviously, this made the U-boat operator's task of searching for stray IF emissions somewhat easier, since as long as a radio was on, it was transmitting on the same parasitic IF.

Update: The IF chosen was somewhat standardized within a country or continent. The U.S.A. and Australia(!) used the same IF's, which varied over the years as circuitry improved. European manufacturers had a different set of IFs that they used. I haven't found any time line that shows when different IFs were active. Nowadays, for broadcast receivers in the US, it's 455kHz. Ham and military radios use different IFs for different frequency bands.

Later, special receivers were built for use on the high seas, that were either shielded, or used a different tuning method to avoid emitting a parasitic radio signal like the superhets.