I did have 2 keys; but of course (s**'s law) I was away from home at the time, and effectively "stranded",
I think you are correct - it needs a good battery to operate reliably.
At home, I then checked the spare key - the battery similarly read 2.9v, and I intend to replace this with a new one in case the fob requires a higher voltage.
By the way, at the time of the failure I was not aware of the advised procedure of pressing the start button with the end of the key fob - I am told that may have turned the ignition on.
I think the learning here is that one should a) not rely on an instrument panel "low battery" warning, b) check occasionally to see that the red led on the fob lights when locking/unlocking, and c) carry a spare battery and a suitable tool carry out a replacement if necessary.