Prius v (lowercase v for the Prius v wagon) key Fob Mystery ?

I know it is something I am doing, but can someone explain to me why occasionally my key fob will not work. I get an error code... far right on the screen that looks like a key with an X? I have my key with me... it will lock and unlock doors but not start the car?
The last time it happened I thought I had figured out the Fob battery was dying. But then 10 minutes later it worked just fine? I also thought the car had somehow become confused as to which key fob was operating the car. MY wife was with me and had gone into to go shopping. Car would not start for me but started when she got in the car with her key fob in her purse. My fob was in my pocket. Could it be the car had somehow selected her fob as the operating fob and went into some kind of security mode when she went in to do shopping without turning off the car? I turned the car off after she was out of range and do not remember if the warning chime had sounded?
Anyone have any idea what happened? It has happened before...
BTW I was almost ready to do the "touch fob to starter button" emergency back up method I had seen on Internet. Would that have started the car? Others have mentioned a warning light for low key fob battery? Where is that?
Thanks.
Love my Prius v (lowercase v for the Prius v wagon) model but there are still some mysterious things that happen with it. Like the "fart noise" that comes from engine about 2 minutes after the car has been turned off. Off course you only notice it if you are still sitting in the turned off car.

 

Noise is from the engine compartment... maybe the passenger side, high up ?
I can replace the battery... but I wanted to make sure it wasn't some weird chain of events I had unintentionally started.
Is there a warning signal of low battery in the fob?