It could be that you previously have somehow said what the sign said and she remembered. Either way, either she can read some or she has an excellent memory!
My younger boy never seemed very gifted, LOL. He would conjugate verbs incorrectly, like say "I dood it" instead of "I did it" even at age 5!
When he was maybe 4 years old, I was rereading whatever book for the millionth time- he had me put the book down (which was a third grade level book) and he recited page after page verbatim with each word correct, without looking at it! LIke a 20 page book. I don't mean, he noticed when mommy skipped a word, he corrected me. I mean, he photographically knew every word on every page without looking at it.
He later got a 165 total IQ on the RIAS and a 99th percentile on the Raven's test, and he's in Mensa. (We haven't done other testing since for us, there is no point).
Anyway... so I suspect you are seeing somebody who is very bright.
