lherlocker

I've got an '07 HR Ambassador with the Allison 3000 series transmission with 60K miles. On a recent trip, about 300 miles into the day's travel, wife and I stopped to make a sandwich at rest area along highway. Coach was left running, in neutral with brake set. Been there a couple of minutes when I noticed the 'Check Trans' light illuminated on dash with '++' showing in transmission selector display area. Light stayed on for approx 15 seconds, went off and selector returned to 'NN'. A minute or so later the warning light returned as well as the '++' display. This cycled several times over the next few minutes. Considering that it was rather late at night, raining and we really needed to get on down the road, I waited until the warnings disappeared and the selector returned to normal, I then hit the 'D' button and we were on the way. Transmission seemed to operated normally for the rest of the evening and in further travels several days later. Any idea as to what the '++' code means and should I be concerned for further transmission damage?