Are you certain the readers were off? The gates can be set to open but
wouldn't normally be powered down unless there's a fault.

I've saved a small fortune out of zone recently because the gates at
stations are open and the Oyster readers don't do anything (no noise,
no LED, nothing recorded on the card) when you touch your card on
them.

There's no point in these endless "remember to touch your card on the
reader even if the gates are open" messages and posters if the readers
are all switched off!

This has happened to me at London Bridge, Morden, South Wimbledon and
Vauxhall (NR) in the past two weeks - I want to give TfL my money but
apparently they don't want it!