I have something similar to this one, but the battery lasts for about a week or two usually.
So when I get caught with it off, I just tell the HB that it's not a watch, it's a magic, telepathic bracellet.
HB: ???
sLICK: Yeah, as a matter of fact it's telling me that you are not wearing any panties right now.
HB: Yes I am!
sLICK: (tapping the watch) Oh man. The damn thing it's an hour ahead again.
HB: Laughing
And they lived happily ever after.
Sometimes she'll just blush or something, and you can triumphantly grin and say "see, I told you so"
That is absolute gold!!! i love it!! i want the watch just for the possibility of that scenario. props for coming up with that