I missed it.
Yesterday evening while I was meditating in yoga class. Jack went over to where the auxiliary potty is kept, lifted up his shirt and pointed at his diaper. Matt said "Belly Button?" and Jack stared at him blankly. Matt said "Potty?" and Jack gestured emphatically. Matt again asked about the belly button with no reaction so he then asked about the potty, not really believing that this was an attempt at actual communication. With his suspicions 'confirmed' he took Jack upstairs to use the potty, where he immediately used the potty.
I guess I'm going to need to start paying attention to his gesturing! I might also need to train him to sit on a big potty so I don't have to carry the little one around in my car- because that's not going to happen.