It's the nature of audio tones. The PL tones are an audible tone that is theoretically "below" what the human ear can hear (but some people can hear the higher ones). If they were hyper-amplified, they'd be the bass notes that you can feel in your chest, even though you don't hear them. At low levels, they "disappear" into the background noise.
However, the radio's amp circuitry, the mics (some of them anyway) and the human voice are all capable of producing those tones, even when it's not intended.
When a radio is using a PL, then it "filters" all audio freqs below some cutoff ABOVE the tone, then injects the tone below it. In the receiver, it intreprets the tone, then filters it back out before audio amplification, to avoid the annoyance of hearing it (cheap radios like scanners probably couldn't reproduce such a low freq with their tiny speakers anyway, so it would just sound bad).
However, radios that DON'T use a tone might or might not filter out freqs in the PL range, so "stray noise" might be transmitted with the modulating signal, or generated in the transmitter itself. If you don't want to see the confusion on the scanner, set it to FM mode instead of PL mode when you know the freq doesn't have a PL, then it won't show anything.
Example:
If I transmit 88.5 PL, my radio might filter "input" modulation (voice) below 150Hz, then inject the 88.5 tone. The receiver that "expects" a tone (any tone, like an amateur repeater) "listens" for the correct tone, then filters the demodulated audio below 300Hz, then (if it's programmed to re-transmit the tone) re-injects the same (or a different) PL.
On the other hand, a "good" receiver might be set to filter (ignore) all demodulated audio below 300Hz (limit of "normal" communications grade receivers/microphones) - thus NO PL is heard/decoded.
Third case, a "cheap" transmitter doesn't limit modulation below 300Hz, and a "cheap" receiver with no audio filters, hears 20Hz to 20,000Hz audio (but can only reproduce 200Hz to 5000Hz) - if it hears a "strong" tone (or lack thereof) it says so (displaying correct PL or none) but if it hears "random" noise generated by the transmitter, it "tries" to decode it, but what it's really decoding is the phase noise generated by the transmitter, the thunder in the background, the car going by, etc. Thus it gets a different "tone" every time it tries.
