Jet, I did work on a KA200 that had a duel transponder failure coming into KLAS.
The seal on the R/H cheek panel was no good and they flew through rain.
Both TDR-90s had water pour out of them when I pulled them.
Unless they have added some wiz-bang feature, it is also possible that A transponder failed. And if they were busy, or ATC didn't call them to point it out, I doubt they would know.
No reply does not mean they were turned off...