I did initially get a bit confused as it seems there are three tones
that the Oyster readers on buses make - the high pitched acceptance
bleep, the lower rejection double bleep, and a special bleep for child
Oyster cards (perhaps also shared with student Oyster cards) - but I
now think some are using an empty Oyster to get past the driver and
evade paying the fare. For some I'd guess this isn't deliberate, but if
bus drivers don't them up on this they won't even know they're doing
it.

I am lucky in that I have only ever had one problem with my Oyster cards on
a bus. My card was being rejected with the error being something like 'out
of zone' despite it being a PAYG only card. Fortunately the driver decided
to believe me and let me on anyway. All I could think was that the reader
hadn't been programmed to understand auto top-up instructions on cards and
was getting confused (it was on a 410 but not one of the usual fleet making
me wonder if was an engineering spare).

G.