well, you CAN tell him you found it and he'll either:
a) Claim it was from a while ago BEFORE he said he'd quit
b) obviously from the neighbors
c) claim a golf buddy gave it to him and he dumped it out
d) accuse YOU of drinking it
e) admit it, saying it was only once, he's back on track
there's a really good reason why we suggest you watch ACTIONS instead of taking mere words to heart. people can SAY anything, hell listen to a polician up for election for two minutes, they'll promise you the moon and the start and endless wealth.
there is a LOT more to quitting than saying I QUIT. in fact some of the best recovery i've ever seen is the QUIET kind, without announcements, proclamations, oaths, or promises. why? cuz they are too busy doing whatever it takes in that day to get sober and stay sober and live a full productive life!!!