So I spooned on a new set of tires last week.
I found that I had 6 or 7 front spokes that were loose; no problem I tightened them up.
On the back I found 4 of the shortest spokes had the heads broken off. Luckily PP had them in stock and I was able to replace them. All of the broken spokes were on the sprocket side of the hub. These are the shortest spokes and I wonder if that contributed to the heads coming off.
I must admit here that I am no wheel expert; I tightened the spokes before we left on our trip and may have over-torqued them. The shortest may have just been under the most stress from this.
Looking into the cost of upgrading the rear spokes set...